Break on syscall?

Is it possible to get gdb to break on entering/exiting a
syscall (rather
than breaking on entering libc or some such)? Does gdb use
ptrace() to
control breakpoints in the debugged code?

--
Alex, homepage: http://www.bennee.com/~a
lex/
Famous, adj.: Conspicuously miserable. -- Ambrose Bierce

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.

> Does gdb use ptrace() to
> control breakpoints in the debugged code?

Breakpoints are inserted usually using PTRACE_POKETEXT.

-- 
Daniel Jacobowitz
CodeSourcery

> Date: Fri, 19 May 2006 08:48:35 -0400
> From: Daniel Jacobowitz <drowfalse.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".

Mark

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 <drowfalse.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 

-- 
Daniel Jacobowitz
CodeSourcery

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 <drowfalse.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 parent’s  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


-=# Paul #=-

PS:  Here in Oregon, rainy months are the norm 

On Fri, May 19, 2006 at 03:09:31PM -0700, PAUL GILLIAM
wrote:
> > 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.

> From the ptrace(2) man page on Linux:
> 
> PTRACE_SYSCALL, PTRACE_SINGLESTEP

That doesn't conflict with what I said.  You can't issue
PTRACE_SYSCALL
and PTRACE_SINGLESTEP at the same time.

> 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
> 

GDB can do vastly better, but it's not a small project.

-- 
Daniel Jacobowitz
CodeSourcery