Modifying ELF files
In my job, we are producing applications and KLM's for our product that require them to be signed so that our installer will recognize and validate our images. The signature is stored in each app as unsigned char signature[40] __attribute__((section(.compsign))); What I need to do is open the file for writing, locate the .compsign section and stuff in the signature, write it out and close the file. (simple ELF manipulation) An 'ls -l' shows the following: % ls compklm.ko -rw-r--r-- 1 pmahan pmahan 125296 Apr 6 22:50 /home/pmahan/temp/compklm.ko When I try to run my program ./signfile --signature=A203239897C8EB360D1EB2C84E8E77B16E5B7C9A compklm.ko open: Text file busy Googling and looking at the kernel sources, it seems that it detects this file contains 'shared text', that is, it is an executable file and does not allow me to open it for writing. I understand (from my google search) this is a means to keep you from shooting yourself in the foot. But there has got to be a way and I really don't want to grovel through the compiler code to find it. I looked at using libelf.so but it also requires that the file be open for writing. So I am kinda of stuck. If I cannot find a quick solution we might need to do all of our signing on our FC11 box which does not have this issue. Thanks for the education I always get from this list, Patrick ___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to freebsd-hackers-unsubscr...@freebsd.org
Re: Modifying ELF files
On Thu, 8 Apr 2010, Patrick Mahan wrote: In my job, we are producing applications and KLM's for our product that require them to be signed so that our installer will recognize and validate our images. The signature is stored in each app as unsigned char signature[40] __attribute__((section(.compsign))); What I need to do is open the file for writing, locate the .compsign section and stuff in the signature, write it out and close the file. (simple ELF manipulation) An 'ls -l' shows the following: % ls compklm.ko -rw-r--r-- 1 pmahan pmahan 125296 Apr 6 22:50 /home/pmahan/temp/compklm.ko When I try to run my program ./signfile --signature=A203239897C8EB360D1EB2C84E8E77B16E5B7C9A compklm.ko open: Text file busy Googling and looking at the kernel sources, it seems that it detects this file contains 'shared text', that is, it is an executable file and does not allow me to open it for writing. My understanding was that ETXTBSY occurs when you attempt to open for writing a file which is actually being executed, i.e. is mapped into some process. I'm not aware that open(2) actually looks at the file itself to see if it is an executable; that would be very surprising to me. What does fstat -m compklm.ko say? What happens if you cp compklm.ko foo.ko and try to sign foo.ko? You should then be able to do mv foo.ko compklm.ko; if compklm.ko is in fact mapped into some process, it will continue to use the original version, which will be kept around (invisibly) until all mappings go away. This is what compilers, install(8), etc, normally do. Does your signfile program do anything with the target file before open(..., O_RDWR)? -- Nate Eldredge n...@thatsmathematics.com ___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to freebsd-hackers-unsubscr...@freebsd.org
Re: Modifying ELF files
On Thu, Apr 08, 2010 at 07:17:46AM -0700, Patrick Mahan wrote: In my job, we are producing applications and KLM's for our product that require them to be signed so that our installer will recognize and validate our images. The signature is stored in each app as unsigned char signature[40] __attribute__((section(.compsign))); What I need to do is open the file for writing, locate the .compsign section and stuff in the signature, write it out and close the file. (simple ELF manipulation) An 'ls -l' shows the following: % ls compklm.ko -rw-r--r-- 1 pmahan pmahan 125296 Apr 6 22:50 /home/pmahan/temp/compklm.ko When I try to run my program ./signfile --signature=A203239897C8EB360D1EB2C84E8E77B16E5B7C9A compklm.ko open: Text file busy Googling and looking at the kernel sources, it seems that it detects this file contains 'shared text', that is, it is an executable file and does not allow me to open it for writing. I understand (from my google search) this is a means to keep you from shooting yourself in the foot. But there has got to be a way and I really don't want to grovel through the compiler code to find it. I looked at using libelf.so but it also requires that the file be open for writing. So I am kinda of stuck. If I cannot find a quick solution we might need to do all of our signing on our FC11 box which does not have this issue. It's not the compiler code you want to find it, but the install(1) program that is used to, well, install files into e.g. /bin, /usr/bin, etc. What it does is create a temporary file in the directory where it wants to place the final file, write into the temporary file, and then, when the file is complete and only when it is complete, it does a rename(2) syscall, moving the temporary file over the real one. If a program (or the kernel) is using the old version of the real file, its inode and its data blocks are still present on the disk and they are only deleted when the last consumer closes the file (or rather, the file descriptor it's holding on that inode). This also guarantees that anyone who tries to open the file will only open it when it's ready, and will not try to execute a partially-written-out executable or something. So, what you need to do if you want to modify a file is create a new one in the same directory (well, it's really on the same filesystem, but the most portable way to ensure that is to use the same directory - unless you require from the user to specify a temporary directory you can use on the same filesystem). Then, read the original file, write into the new one, and when you're ready, do a rename(tempfile, realfile). Hope that helps. G'luck, Peter -- Peter Pentchev r...@space.bgr...@ringlet.netr...@freebsd.org PGP key:http://people.FreeBSD.org/~roam/roam.key.asc Key fingerprint FDBA FD79 C26F 3C51 C95E DF9E ED18 B68D 1619 4553 signature.asc Description: Digital signature
Re: Modifying ELF files
On Thu, 8 Apr 2010, Patrick Mahan wrote: In my job, we are producing applications and KLM's for our product that require them to be signed so that our installer will recognize and validate our images. The signature is stored in each app as unsigned char signature[40] __attribute__((section(.compsign))); What I need to do is open the file for writing, locate the .compsign section and stuff in the signature, write it out and close the file. (simple ELF manipulation) An 'ls -l' shows the following: % ls compklm.ko -rw-r--r-- 1 pmahan pmahan 125296 Apr 6 22:50 /home/pmahan/temp/compklm.ko When I try to run my program ./signfile --signature=A203239897C8EB360D1EB2C84E8E77B16E5B7C9A compklm.ko open: Text file busy Googling and looking at the kernel sources, it seems that it detects this file contains 'shared text', that is, it is an executable file and does not allow me to open it for writing. My understanding was that ETXTBSY occurs when you attempt to open for writing a file which is actually being executed, i.e. is mapped into some process. I'm not aware that open(2) actually looks at the file itself to see if it is an executable; that would be very surprising to me. What does fstat -m compklm.ko say? % fstat -m compklm.ko USER CMD PID FD MOUNT INUM MODE SZ|DV R/W NAME % What happens if you cp compklm.ko foo.ko and try to sign foo.ko? You should then be able to do mv foo.ko compklm.ko; if compklm.ko is in fact mapped into some process, it will continue to use the original version, which will be kept around (invisibly) until all mappings go away. This is what compilers, install(8), etc, normally do. Does your signfile program do anything with the target file before open(..., O_RDWR)? I've just found my problem. We have a wrapper program that basically handles parsing command line options and is suppose to adjust the argv[] array so that it only contains the remaining non-option targets starting at index zero. So I am doing 'open(argv[0], O_RDWR, 0)' expecting it to be the .ko file. Turns out it was not operating as described (whipping post to be erected later); so argv[0] actually pointed at the operating program, not the first target past the cmd line options. *-) Mystery solved. Thanks, Patrick ___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to freebsd-hackers-unsubscr...@freebsd.org
