One more thought about vim race condition during file saving
Did you think about use leases to exclusively lock file during save? I mean fcntl() with F_SETLEASE. It acquires mandatory lock and doesn't break hard links, which you afraid of. Processes which try to read file during saving are blocked on open() until lease is removed, so it doesn't change behaviour from other processes' point of view. On the other hand, it is not possible to obtain write lease on file already opened by another process, and in such a situation (which IMHO shouldn't happen often with files which are edited by hand) vim should retry acquiring lease after short sleep or fallback to current behaviour. This method was originally developed for use in samba, but it is available in Linux kernel since 2002 (or even earlier), so it could be said that it is standard. Certainly, it is Linux specific feature, but if it is available, why shouldn't it be used. Any pros or cons? Regards. --~--~-~--~~~---~--~~ You received this message from the "vim_dev" maillist. For more information, visit http://www.vim.org/maillist.php -~--~~~~--~~--~--~---
Re: Race condition during file saving
>> If you're writing to a file that another program critically needs >> /that's/ your problem. > > Configuration file, for example, is critically for almost every daemon. > Do you think that it is only my problem? No, it is very real scenario > which could happen to everyone who use vim. He didn't mean it's your problem personally and that no other user would experience it. He meant it's your problem as the user, not the editor's problem. If you edit your config files, most likely you need to SIGHUP the daemon to reread it anyway. And if you don't, most likely the daemon is aware of the somewhat dumb practice of reading its config files at unpredictable intervals, and provides a device, such as crontab -e, to avoid problems, as has already been mentioned. Playing games moving temporary files around for the purpose of keeping daemons happy is not an editor's job, but the user's, or a utility such as crontab's. Having the editor write to a temporary file and move it into place has other worse effects as have already been mentioned (breaking of links, use of file descriptors by calling processes such as crontab -e, etc.). Indeed, other software sometimes expects the editor to overwrite the original file, which implies a time with an empty or partially written file. So although the current behaviour isn't really ideal, neither are the alternatives, and in fact, the alternatives are probably worse. It could be another option, I suppose, or incorporated into an existing one (it is only possible if writebackup and backup are unset or copybackup is yes, or there is a time of nonexistence of the file, which is just as bad), but is it worth the bother? How often are people actually bitten by this? Very rarely, I think, and the consequences are far from catastrophic when it happens. It couldn't be the default due to the other negative side effects of the method, too, so would have to be set when appropriate, which means you might as well use some other tool to do the job. Ben. --~--~-~--~~~---~--~~ You received this message from the "vim_dev" maillist. For more information, visit http://www.vim.org/maillist.php -~--~~~~--~~--~--~---
Re: Race condition during file saving
On Mon, Jan 5, 2009 at 00:06, Adam Osuchowski wrote: > Nikolai Weibull wrote: >> Either way, I really don't think we have a problem to fix. > So what is your advice? Ignore it? It's very comfortable to call flows > features, we have perfect situation and don't need to worry about > consequences. ? >> If you're writing to a file that another program critically needs >> /that's/ your problem. > Configuration file, for example, is critically for almost every daemon. > Do you think that it is only my problem? No, it is very real scenario > which could happen to everyone who use vim. That's what crontab -e is for, among other things. --~--~-~--~~~---~--~~ You received this message from the "vim_dev" maillist. For more information, visit http://www.vim.org/maillist.php -~--~~~~--~~--~--~---
Re: Race condition during file saving
Charles E. Campbell, Jr. wrote: > Likely areas for problems like this concern cooperative editing (ie. > multiple people editing the same file) and editing log files (or other > files which are potentially being written to by some other program). > Vim isn't designed for cooperative editing; I seem to recall it being on > a wishlist, though. Editing log files is problematic because they > generally aren't using mandatory file locking. I didn't tell about multiple people editing the same file or editing log files. I told about _ONE_ person editing file which could be read by another process at the same time. Isn't vim designed for it too? Don't think so. > To avoid the need for cooperative editing, use cvs/git/etc and use separate > copies and repositories. And keep all /etc files in cvs repo? It's only pinning the blame on another application (cvs, in this case). --~--~-~--~~~---~--~~ You received this message from the "vim_dev" maillist. For more information, visit http://www.vim.org/maillist.php -~--~~~~--~~--~--~---
Re: Race condition during file saving
Nikolai Weibull wrote: > Either way, I really don't think we have a problem to fix. So what is your advice? Ignore it? It's very comfortable to call flows features, we have perfect situation and don't need to worry about consequences. > If you're writing to a file that another program critically needs > /that's/ your problem. Configuration file, for example, is critically for almost every daemon. Do you think that it is only my problem? No, it is very real scenario which could happen to everyone who use vim. --~--~-~--~~~---~--~~ You received this message from the "vim_dev" maillist. For more information, visit http://www.vim.org/maillist.php -~--~~~~--~~--~--~---
Re: Race condition during file saving
Nikolai Weibull wrote: > On Sun, Jan 4, 2009 at 23:19, Charles E. Campbell, Jr. > wrote: > > >> Nikolai Weibull wrote: >> > > >>> No, I mean both O_EXCL (so that a file hasn't been created in between >>> the time the original file has been renamed and the new one opened - a >>> case so far not mentioned (or?)) and flock:ing it (but, as I >>> understand from the man page, this doesn't seem to be very interesting >>> either). >>> > > >> If you're talking Unix/Linux/etc -- flock is a cooperative file locking >> mechanism, and processes which ignore it are not impeded. >> > > Just to make sure, did you read the part where I wrote "[flock] > doesn't seem to be very interesting either"? > > I just want to know what your intentions were. > > Was it simply to fill in extra information about why I wrote that? > > Or did it seem like I didn't know what I was talking about and you > wrote this reply to correct me? > > I'm finding that I get a lot of replys, especially at work, where I > get the feeling that my mail hasn't really been read through. A > classic is where you ask someone two questions and you get a reply > with an answer for the first one. Or where you answer two questions > and they restate the second one in their reply. > > Either way, I really don't think we have a problem to fix. I /want/ > to be able to read what a program is writing to a file, mid-write. > Generally, when a long process is executing I want to be able to tell > what's being written. If you're writing to a file that another > program critically needs /that's/ your problem. Not whether you made > sure to set up some intricate locking mechanism or not. > Nikolai -- you're getting a bit sensitive here -- it was "simply to fill in extra information...", as you hadn't explained why flock "wasn't interesting." Plus I added the mandatory kernel-enforced locking information. Likely areas for problems like this concern cooperative editing (ie. multiple people editing the same file) and editing log files (or other files which are potentially being written to by some other program). Vim isn't designed for cooperative editing; I seem to recall it being on a wishlist, though. Editing log files is problematic because they generally aren't using mandatory file locking. Personally, I agree with you that there's not a problem to fix; at most, there's new behavior (that wishlist stuff) that someone may want. To avoid the need for cooperative editing, use cvs/git/etc and use separate copies and repositories. Editing log files is likely to remain problematic and requires more than just vim to change (unless the logging process already uses flock, perhaps I'll look into it sometime). Regards, Chip Campbell --~--~-~--~~~---~--~~ You received this message from the "vim_dev" maillist. For more information, visit http://www.vim.org/maillist.php -~--~~~~--~~--~--~---
Re: Race condition during file saving
On Sun, Jan 4, 2009 at 23:19, Charles E. Campbell, Jr. wrote: > Nikolai Weibull wrote: >> No, I mean both O_EXCL (so that a file hasn't been created in between >> the time the original file has been renamed and the new one opened - a >> case so far not mentioned (or?)) and flock:ing it (but, as I >> understand from the man page, this doesn't seem to be very interesting >> either). > If you're talking Unix/Linux/etc -- flock is a cooperative file locking > mechanism, and processes which ignore it are not impeded. Just to make sure, did you read the part where I wrote "[flock] doesn't seem to be very interesting either"? I just want to know what your intentions were. Was it simply to fill in extra information about why I wrote that? Or did it seem like I didn't know what I was talking about and you wrote this reply to correct me? I'm finding that I get a lot of replys, especially at work, where I get the feeling that my mail hasn't really been read through. A classic is where you ask someone two questions and you get a reply with an answer for the first one. Or where you answer two questions and they restate the second one in their reply. Either way, I really don't think we have a problem to fix. I /want/ to be able to read what a program is writing to a file, mid-write. Generally, when a long process is executing I want to be able to tell what's being written. If you're writing to a file that another program critically needs /that's/ your problem. Not whether you made sure to set up some intricate locking mechanism or not. --~--~-~--~~~---~--~~ You received this message from the "vim_dev" maillist. For more information, visit http://www.vim.org/maillist.php -~--~~~~--~~--~--~---
Re: Race condition during file saving
Nikolai Weibull wrote: > On Sun, Jan 4, 2009 at 16:26, Adam Osuchowski wrote: > > >> Nikolai Weibull wrote: >> > > >>> I haven't really understood what the problem is (I don't believe that >>> there actually is one), >>> > > >> There is a non-zero time period between open file and write complete >> content. Because vim truncate file while opening (O_TRUNC), another >> process which would like to read it can hit in the moment when file will >> be empty. >> > > Yeah, so? That's not a bug. File systems are, generally, not > databases with ACID properties. They don't provide transactions (at > the level we're discussing). > > >>> but would opening the file exclusively solve the problem? >>> > > >> Do you mean O_EXCL flag to open syscall? It doesn't protect against >> such a situation. It only guarantee that file doesn't exist prior to >> call open in atomic manner. >> > > No, I mean both O_EXCL (so that a file hasn't been created in between > the time the original file has been renamed and the new one opened - a > case so far not mentioned (or?)) and flock:ing it (but, as I > understand from the man page, this doesn't seem to be very interesting > either). > If you're talking Unix/Linux/etc -- flock is a cooperative file locking mechanism, and processes which ignore it are not impeded. Look into mandatory file locking (the mand option to mount). Mandatory file locking is kernel enforced, and not even root apparently can force its way through. Mandatory file locking by vim would not be practical, as it generally requires root access to set up, and only involves files on the mandatory-filelocking mounted device. Regards, Chip Campbell --~--~-~--~~~---~--~~ You received this message from the "vim_dev" maillist. For more information, visit http://www.vim.org/maillist.php -~--~~~~--~~--~--~---
Re: Race condition during file saving
On Sun, Jan 4, 2009 at 16:26, Adam Osuchowski wrote: > Nikolai Weibull wrote: >> I haven't really understood what the problem is (I don't believe that >> there actually is one), > There is a non-zero time period between open file and write complete > content. Because vim truncate file while opening (O_TRUNC), another > process which would like to read it can hit in the moment when file will > be empty. Yeah, so? That's not a bug. File systems are, generally, not databases with ACID properties. They don't provide transactions (at the level we're discussing). >> but would opening the file exclusively solve the problem? > Do you mean O_EXCL flag to open syscall? It doesn't protect against > such a situation. It only guarantee that file doesn't exist prior to > call open in atomic manner. No, I mean both O_EXCL (so that a file hasn't been created in between the time the original file has been renamed and the new one opened - a case so far not mentioned (or?)) and flock:ing it (but, as I understand from the man page, this doesn't seem to be very interesting either). --~--~-~--~~~---~--~~ You received this message from the "vim_dev" maillist. For more information, visit http://www.vim.org/maillist.php -~--~~~~--~~--~--~---
Re: Race condition during file saving
Nikolai Weibull wrote: > I haven't really understood what the problem is (I don't believe that > there actually is one), There is a non-zero time period between open file and write complete content. Because vim truncate file while opening (O_TRUNC), another process which would like to read it can hit in the moment when file will be empty. > but would opening the file exclusively solve the problem? Do you mean O_EXCL flag to open syscall? It doesn't protect against such a situation. It only guarantee that file doesn't exist prior to call open in atomic manner. --~--~-~--~~~---~--~~ You received this message from the "vim_dev" maillist. For more information, visit http://www.vim.org/maillist.php -~--~~~~--~~--~--~---
Re: Race condition during file saving
On Sun, Jan 4, 2009 at 15:57, Adam Osuchowski wrote:
> open("testfile", O_WRONLY|O_CREAT|O_TRUNC, 0600) = 3
I haven't really understood what the problem is (I don't believe that
there actually is one), but would opening the file exclusively solve
the problem?
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Re: Race condition during file saving
Bram Moolenaar wrote:
> That's all taken care of when 'backupcopy' is "auto". If you want the
> original file to always exist set 'backupcopy' to "yes". Saving files
> will be slower then, since Vim needs to both write a copy and write the
> actual file.
Not quite. Of course, with 'backupcopy' set to "yes", there are not
moment when another process find file missing, but still file may be empty
or not completely written:
open("testfile", O_WRONLY|O_CREAT|O_TRUNC, 0600) = 3
write(3, "test test test\n"..., 15) = 15
fsync(3) = 0
close(3) = 0
> Since you are overwriting the file there always is a moment it's empty.
Unless use of rename() syscall, which replace it atomically (with exact
to hard link cases).
I know, that these problems result from flawed POSIX file system syscalls
behaviour, but IMHO it may be made better than it is done now.
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Re: Race condition during file saving
Matt Wozniski wrote: > rename(2) doesn't do everything needed. Right, but current behaviour is even worse. We can't protect if somebody create file while vim saves it due to system limitations, but we can protect against completely lack of file or situation when it is partially written. > rename() would break the link, which probably isn't what you want an > editor to do... Right, but definitely there are fewer multi hard linked files than singles. Again, we can't protect against such situation because of POSIX syscalls nature, so maybe vim should identify if there is hard link and unless, it will do atomic file replacement. I know, there is another race condition (between stat() and rename()) but it is more unlikely case. --~--~-~--~~~---~--~~ You received this message from the "vim_dev" maillist. For more information, visit http://www.vim.org/maillist.php -~--~~~~--~~--~--~---
Re: Race condition during file saving
Matt Wozniski wrote: > On Sat, Jan 3, 2009 at 11:27 PM, Adam Osuchowski wrote: > > > > Cases like mbox files are not so rare. There are many examples of > > simultaneously access to single file, but problem exists even without > > concurrent modification. > > > > A simple example: editing config file for some daemon. When vim > > truncates this file and program read it at the same time (because, > > for example, it will be restarted by cron, other administrator or even > > by itself) it will be problematic situation. The same matter is if > > a program will be run from cron or other program (for example, procmail > > from sendmail). Recover option in vim does not help here. Do you suggest > > turn off all processes during editing config files? > > > > Vim is used on multiuser and multiprocess systems, so limiting the working > > users or simultaneously processes to single one is misunderstanding. > > Why do you disrespect problem, especially if there is solution in the > > form of proper use of rename(2) syscall, which I mentioned about? > > rename(2) doesn't do everything needed. What if the file being > written is a symlink, or a hard link? rename() would break the link, > which probably isn't what you want an editor to do... That's all taken care of when 'backupcopy' is "auto". If you want the original file to always exist set 'backupcopy' to "yes". Saving files will be slower then, since Vim needs to both write a copy and write the actual file. Since you are overwriting the file there always is a moment it's empty. -- If you don't get everything you want, think of everything you didn't get and don't want. /// Bram Moolenaar -- b...@moolenaar.net -- http://www.Moolenaar.net \\\ ///sponsor Vim, vote for features -- http://www.Vim.org/sponsor/ \\\ \\\download, build and distribute -- http://www.A-A-P.org/// \\\help me help AIDS victims -- http://ICCF-Holland.org/// --~--~-~--~~~---~--~~ You received this message from the "vim_dev" maillist. For more information, visit http://www.vim.org/maillist.php -~--~~~~--~~--~--~---
Re: Race condition during file saving
On Sat, Jan 3, 2009 at 11:27 PM, Adam Osuchowski wrote: > > Cases like mbox files are not so rare. There are many examples of > simultaneously access to single file, but problem exists even without > concurrent modification. > > A simple example: editing config file for some daemon. When vim > truncates this file and program read it at the same time (because, > for example, it will be restarted by cron, other administrator or even > by itself) it will be problematic situation. The same matter is if > a program will be run from cron or other program (for example, procmail > from sendmail). Recover option in vim does not help here. Do you suggest > turn off all processes during editing config files? > > Vim is used on multiuser and multiprocess systems, so limiting the working > users or simultaneously processes to single one is misunderstanding. > Why do you disrespect problem, especially if there is solution in the > form of proper use of rename(2) syscall, which I mentioned about? rename(2) doesn't do everything needed. What if the file being written is a symlink, or a hard link? rename() would break the link, which probably isn't what you want an editor to do... ~Matt --~--~-~--~~~---~--~~ You received this message from the "vim_dev" maillist. For more information, visit http://www.vim.org/maillist.php -~--~~~~--~~--~--~---
Re: Race condition during file saving
Tony Mechelynck wrote: > I don't know. There're only one keyboard and one display on this > machine, and I try to avoid having several programs modify a single file > simultaneously outside each other's knowledge. The rare case is > /var/spool/mail/root which is appended to by my cron jobs, and truncated > by SeaMonkey after "downloading" the mail to its own mailbox in its > profile. Vim doesn't intervene there. > > Vim will try to detect when its editfile has been modified by another > program, but it is not meant to be used in an environment where anything > can be modified simultaneously by any number of actors. If something > goes wrong, you can try to |recover|. Cases like mbox files are not so rare. There are many examples of simultaneously access to single file, but problem exists even without concurrent modification. A simple example: editing config file for some daemon. When vim truncates this file and program read it at the same time (because, for example, it will be restarted by cron, other administrator or even by itself) it will be problematic situation. The same matter is if a program will be run from cron or other program (for example, procmail from sendmail). Recover option in vim does not help here. Do you suggest turn off all processes during editing config files? Vim is used on multiuser and multiprocess systems, so limiting the working users or simultaneously processes to single one is misunderstanding. Why do you disrespect problem, especially if there is solution in the form of proper use of rename(2) syscall, which I mentioned about? --~--~-~--~~~---~--~~ You received this message from the "vim_dev" maillist. For more information, visit http://www.vim.org/maillist.php -~--~~~~--~~--~--~---
Re: Race condition during file saving
On 04/01/09 04:19, Adam Osuchowski wrote:
> Tony Mechelynck wrote:
>> See
>> :help backup
>> :help 'backup'
>> :help 'writebackup'
>> :help 'backupcopy'
>> :help timestamp
>
> I try different settings of this variables and there was always the same
> situation:
>
> open("testfile", O_WRONLY|O_CREAT|O_TRUNC, 0600) = 3
> write(3, "test test test\n"..., 15) = 15
>
> So, still there are points in time when file could be empty (after opening
> with O_TRUNC) or partially written (between multiple write syscalls).
> Could you give me a concrete example of values of these settings, which
> could prevent such situations?
I don't know. There're only one keyboard and one display on this
machine, and I try to avoid having several programs modify a single file
simultaneously outside each other's knowledge. The rare case is
/var/spool/mail/root which is appended to by my cron jobs, and truncated
by SeaMonkey after "downloading" the mail to its own mailbox in its
profile. Vim doesn't intervene there.
Vim will try to detect when its editfile has been modified by another
program, but it is not meant to be used in an environment where anything
can be modified simultaneously by any number of actors. If something
goes wrong, you can try to |recover|.
Best regards,
Tony.
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-- D. Letterman
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Re: Race condition during file saving
Tony Mechelynck wrote:
> See
> :help backup
> :help 'backup'
> :help 'writebackup'
> :help 'backupcopy'
> :help timestamp
I try different settings of this variables and there was always the same
situation:
open("testfile", O_WRONLY|O_CREAT|O_TRUNC, 0600) = 3
write(3, "test test test\n"..., 15) = 15
So, still there are points in time when file could be empty (after opening
with O_TRUNC) or partially written (between multiple write syscalls).
Could you give me a concrete example of values of these settings, which
could prevent such situations?
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Re: Race condition during file saving
On 03/01/09 22:03, Adam Osuchowski wrote:
> There is a race condition in vim 7.2 (and probably in earlier too) on POSIX
> platforms. Below, there is fragment of strace output related to this problem.
>
>
> stat64("testfile~", 0xbfc35dbc) = -1 ENOENT (No such file or
> directory)
> stat64("testfile", {st_mode=S_IFREG|0600, st_size=12, ...}) = 0
> unlink("testfile~") = -1 ENOENT (No such file or
> directory)
> rename("testfile", "testfile~") = 0
> open("testfile", O_WRONLY|O_CREAT|O_TRUNC, 0600) = 3
> write(3, "test test test\n"..., 15) = 15
> fsync(3)= 0
> stat64("testfile", {st_mode=S_IFREG|0600, st_size=15, ...}) = 0
> stat64("testfile", {st_mode=S_IFREG|0600, st_size=15, ...}) = 0
> close(3)= 0
>
>
> Problematic situation take place during file saving between rename and open,
> open and write as well as write and close syscalls. There are points at
> which another process attempting to access file may run into trouble.
> There is, for example, possibility to:
>
> - if application, which expects existence of file, would try to open it
>between vim rename and open syscalls, it will fail due to lack of this
>file,
>
> - if other process creates file with the same name between vim rename and
>open syscalls, it will be overriden by vim (it works with symlinks too,
>so it can be used by attacker to damage other files),
>
> - if application will read file while vim will write to it, the contents
>may be badly read due to temporarily partially record.
>
> Vim rather should create new, its own, temporary file with unique name,
> write content, close it and then, atomically rename it to original name.
>
> Regards.
See
:help backup
:help 'backup'
:help 'writebackup'
:help 'backupcopy'
:help timestamp
Best regards,
Tony.
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ARTHUR: I am your king!
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Race condition during file saving
There is a race condition in vim 7.2 (and probably in earlier too) on POSIX
platforms. Below, there is fragment of strace output related to this problem.
stat64("testfile~", 0xbfc35dbc) = -1 ENOENT (No such file or directory)
stat64("testfile", {st_mode=S_IFREG|0600, st_size=12, ...}) = 0
unlink("testfile~") = -1 ENOENT (No such file or directory)
rename("testfile", "testfile~") = 0
open("testfile", O_WRONLY|O_CREAT|O_TRUNC, 0600) = 3
write(3, "test test test\n"..., 15) = 15
fsync(3)= 0
stat64("testfile", {st_mode=S_IFREG|0600, st_size=15, ...}) = 0
stat64("testfile", {st_mode=S_IFREG|0600, st_size=15, ...}) = 0
close(3)= 0
Problematic situation take place during file saving between rename and open,
open and write as well as write and close syscalls. There are points at
which another process attempting to access file may run into trouble.
There is, for example, possibility to:
- if application, which expects existence of file, would try to open it
between vim rename and open syscalls, it will fail due to lack of this
file,
- if other process creates file with the same name between vim rename and
open syscalls, it will be overriden by vim (it works with symlinks too,
so it can be used by attacker to damage other files),
- if application will read file while vim will write to it, the contents
may be badly read due to temporarily partially record.
Vim rather should create new, its own, temporary file with unique name,
write content, close it and then, atomically rename it to original name.
Regards.
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