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Re: Break on syscall?
On Fri, 2006-05-19 at 18:05 -0400, Daniel Jacobowitz wrote:
> On Fri, May 19, 2006 at 11:16:15PM +0200, Mark Kettenis wrote:
> > > Date: Fri, 19 May 2006 08:48:35 -0400
> > > From: Daniel Jacobowitz <drow@false.org>
> > >
> > > On Fri, May 19, 2006 at 11:15:30AM +0100, Alex Bennee wrote:
> > > > Is it possible to get gdb to break on entering/exiting a syscall (rather
> > > > than breaking on entering libc or some such)?
> > >
> > > This is not supported.
> >
> > But I think it would be nice if we would support something like "catch
> > syscall", just like we support "catch fork".
>
> Yes, probably. I think I even started work on this once. It's just a
> bit trickier. Not only do you want to be able to decode arguments, but
> there are other problems... for example, I think procfs allows it, but
> traditionally ptrace has no way to request a single step and stop if
> entering a syscall, so you'd need an arch hook to detect it to handle
> that case.
>
> A nice project for some rainy month :-)
>
>From the ptrace(2) man page on Linux:
PTRACE_SYSCALL, PTRACE_SINGLESTEP
Restarts the stopped child as for PTRACE_CONT, but arranges for
the child to be stopped at the next entry to or exit from a sys-
tem call, or after execution of a single instruction, respec-
tively. (The child will also, as usual, be stopped upon receipt
of a signal.) From the parentâs perspective, the child will
appear to have been stopped by receipt of a SIGTRAP. So, for
PTRACE_SYSCALL, for example, the idea is to inspect the argu-
ments to the system call at the first stop, then do another
PTRACE_SYSCALL and inspect the return value of the system call
at the second stop. (addr is ignored.)
The 'ltrace' utility uses this to trace system calls. It uses a sleazy
table (/etc/ltrace.cfg) to find out about their arguments... GDB should
be able to do a much better job, although matching syscall numbers to
their associated library routines would be a challenge (at least for me
8-)
-=# Paul #=-
PS: Here in Oregon, rainy months are the norm 8-)