On Wed, Aug 19, 2015 at 04:59:38PM -0400, Mike Frysinger wrote:
> On 19 Aug 2015 22:36, Dmitry V. Levin wrote:
> > On Wed, Aug 19, 2015 at 03:17:39PM -0400, Mike Frysinger wrote:
> > > some syscalls have arguments that are read/write. for example,
> > > getsockopt
> > > passes in a pointer to a length that has to be set correctly first, and
> > > then
> > > the kernel will adjust it when returning. being able to see both values
> > > is
> > > important when getting an error so you can see what the user sent up and
> > > what
> > > the kernel sent back.
> > >
> > > are there examples in strace to look at ? perhaps something like:
> > > getsockopt(4, SOL_IP, IPT_SO_GET_ENTRIES, "...", [2900->2804]) = 0
> > > and when you get an error it'd be:
> > > getsockopt(4, SOL_IP, IPT_SO_GET_ENTRIES, 0x12345, [2900->2900]) = EINVAL
> >
> > I think there is no need to print it in case of an error,
> > when it's known to be unchanged.
> >
> > We have several examples in the code:
> > $ git grep '[^a-z] => ' *.c
> > mtd.c: tprints(" => ");
> > mtd.c: tprints(" => ");
> > sendfile.c: tprints(" => ");
> > sendfile.c: tprints(" => ");
> > sock.c: tprints(" => ");
> > v4l2.c: tprints(exiting(tcp) && code != VIDIOC_G_FMT ?
> > " => " : ", ");
> > v4l2.c: tprints(exiting(tcp) && code == VIDIOC_S_PARM ? " => {"
> > : ", {");
> > v4l2.c: tprints(exiting(tcp) ? " => " : ", {id=");
> > v4l2.c: tprints(code != VIDIOC_G_EXT_CTRLS && exiting(tcp) ? "
> > => " : ", ");
> >
> > The most recent one is from commits v4.10-319-g22f8b27 and
> > v4.10-320-g4918285.
>
> output format looks fine.[before => after] might look better, but [before] => [after] is easier to implement. > but there's another wrinkle here :). sendfile is > easy -- the leading args are input only, and the last one is input/output, > so incremental output is not an issue. getsockopt is inputs, then output, > then input/output. so i can't print the last input in the entering code path > and then print the output in the exiting code path. i need to read the curr > value, save it in the tcp structure somehow (?), and then read that back out > in the exiting code path. This is not the first time a necessity to save some data between entering and exiting stages of a syscall parser is mentioned. For example, there was a discussion back in July about adding a private data field to struct tcb: http://sourceforge.net/p/strace/mailman/message/34262875/ However, none of those ideas have been implemented yet. -- ldv
pgpiWIY1g_K1w.pgp
Description: PGP signature
------------------------------------------------------------------------------
_______________________________________________ Strace-devel mailing list Strace-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/strace-devel
