Re: [PATCH 1/3] [gdb/tdep] Add syscall number cache

[John Baldwin <jhb@FreeBSD.org>]

On 11/22/23 1:10 AM, Tom de Vries wrote:
> When running test-case gdb.base/catch-syscall.exp on powerpc64le-linux, we run
> into an xfail:
> ...
> (gdb) catch syscall execve^M
> Catchpoint 18 (syscall 'execve' [11])^M
> (gdb) PASS: gdb.base/catch-syscall.exp: execve: \
>    catch syscall with arguments (execve)
>    ...
> continue^M
> Continuing.^M
> ^M
> Catchpoint 18 (call to syscall execve), 0x00007ffff7d7f18c in execve () from \
>    /lib64/libc.so.6^M
> (gdb) PASS: gdb.base/catch-syscall.exp: execve: program has called execve
> continue^M
> Continuing.^M
> process 60484 is executing new program: catch-syscall^M
> ^M
> Breakpoint 17, main (argc=1, argv=0x7fffffffe618) at catch-syscall.c:54^M
> 54              char buf1[2] = "a";^M
> (gdb) XFAIL: gdb.base/catch-syscall.exp: execve: syscall execve has returned
> ...
> 
> The problem is that the catchpoint "(return from syscall execve)" doesn't
> trigger.
> 
> This is caused by ppc_linux_get_syscall_number returning 0 at execve
> syscall-exit-stop, while it should return 11.
> 
> This is a problem that was fixed in linux kernel version v5.19, by commit
> ec6d0dde71d7 ("powerpc: Enable execve syscall exit tracepoint"), but the
> machine I'm running the tests on has v4.18.0.
> 
> An approach was discussed in the PR where ppc_linux_get_syscall_number would
> try to detect an execve syscall-exit-stop based on the register state, but
> that was considered too fragile.
> 
> Fix this by caching the syscall number at syscall-enter-stop, and reusing it
> at syscall-exit-stop.
> 
> This is sufficient to stop triggering the xfail, so remove it.
> 
> It's good to point out that this doesn't always eliminate the need to get the
> syscall number at a syscall-exit-stop.
> 
> The test-case has an example called mid-vfork, where we do:
> - catch vfork
> - continue
> - catch syscall
> - continue.
> 
> The following things happen:
> - the "catch vfork" specifies that we capture the PTRACE_EVENT_VFORK event.
> - the first continue runs into the event
> - the "catch syscall" specifies that we capture syscall-enter-stop and
>    syscall-exit-stop events.
> - the second continue runs into the syscall-exit-stop.  At that point there's
>    no syscall number value cached, because no corresponding syscall-enter-stop
>    was observed.
> 
> We can address this issue somewhat by translating events into syscalls.  A
> followup patch in this series use this approach (though not for vfork).
> 
> PR tdep/28623
> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28623
> ---
>   gdb/linux-nat.c                          | 45 ++++++++++++++++++++++--
>   gdb/linux-nat.h                          |  3 ++
>   gdb/testsuite/gdb.base/catch-syscall.exp |  8 +----
>   3 files changed, 46 insertions(+), 10 deletions(-)
> 
> diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
> index 7b0562cf89b..ab6eadd557d 100644
> --- a/gdb/linux-nat.c
> +++ b/gdb/linux-nat.c
> @@ -1508,6 +1508,17 @@ linux_resume_one_lwp_throw (struct lwp_info *lp, int step,
>     else
>       lp->stop_pc = 0;
>   
> +  if (catch_syscall_enabled () > 0)
> +    {
> +      /* Function inf_ptrace_target::resume uses PT_SYSCALL.  */
> +    }
> +  else
> +    {
> +      /* Function inf_ptrace_target::resume uses PT_CONTINUE.
> +	 Invalidate syscall_number cache.  */
> +      lp->syscall_number = -1;
> +    }
> +
>     linux_target->low_prepare_to_resume (lp);
>     linux_target->low_resume (lp->ptid, step, signo);
>   
> @@ -1762,7 +1773,26 @@ linux_handle_syscall_trap (struct lwp_info *lp, int stopping)
>     struct target_waitstatus *ourstatus = &lp->waitstatus;
>     struct gdbarch *gdbarch = target_thread_architecture (lp->ptid);
>     thread_info *thread = linux_target->find_thread (lp->ptid);
> +
> +  enum target_waitkind new_syscall_state
> +    = (lp->syscall_state == TARGET_WAITKIND_SYSCALL_ENTRY
> +       ? TARGET_WAITKIND_SYSCALL_RETURN
> +       : TARGET_WAITKIND_SYSCALL_ENTRY);
> +
>     int syscall_number = (int) gdbarch_get_syscall_number (gdbarch, thread);
> +  if (new_syscall_state == TARGET_WAITKIND_SYSCALL_RETURN
> +      && lp->syscall_number != -1
> +      && lp->syscall_number != syscall_number)
> +    {
> +      /* Calling gdbarch_get_syscall_number for TARGET_WAITKIND_SYSCALL_RETURN
> +	 is unreliable on some targets for some syscalls, use the syscall
> +	 detected at TARGET_WAITKIND_SYSCALL_ENTRY instead.  */
> +      linux_nat_debug_printf
> +	(_("Using syscall number %d supplied by syscall_number cache instead"
> +	   " of %d supplied by architecture hook"),
> +	       lp->syscall_number, syscall_number);
> +      syscall_number = lp->syscall_number;
> +    }
>   
>     if (stopping)
>       {
> @@ -1801,9 +1831,18 @@ linux_handle_syscall_trap (struct lwp_info *lp, int stopping)
>        the user could install a new catchpoint for this syscall
>        between syscall enter/return, and we'll need to know to
>        report a syscall return if that happens.  */
> -  lp->syscall_state = (lp->syscall_state == TARGET_WAITKIND_SYSCALL_ENTRY
> -		       ? TARGET_WAITKIND_SYSCALL_RETURN
> -		       : TARGET_WAITKIND_SYSCALL_ENTRY);
> +  lp->syscall_state = new_syscall_state;
> +
> +  if (lp->syscall_state == TARGET_WAITKIND_SYSCALL_ENTRY)
> +    {
> +      /* Save to use in TARGET_WAITKIND_SYSCALL_RETURN.  */
> +      lp->syscall_number = syscall_number;
> +    }
> +  else
> +    {
> +      /* Reset to prevent stale values.  */
> +      lp->syscall_number = -1;
> +    }
>   
>     if (catch_syscall_enabled ())
>       {
> diff --git a/gdb/linux-nat.h b/gdb/linux-nat.h
> index 428bb9f1628..b17037400a3 100644
> --- a/gdb/linux-nat.h
> +++ b/gdb/linux-nat.h
> @@ -277,6 +277,9 @@ struct lwp_info : intrusive_list_node<lwp_info>
>        - TARGET_WAITKIND_SYSCALL_RETURN */
>     enum target_waitkind syscall_state;
>   
> +  /* Syscall number corresponding to syscall_state.  */
> +  int syscall_number = -1;
> +
>     /* The processor core this LWP was last seen on.  */
>     int core = -1;
>   
> diff --git a/gdb/testsuite/gdb.base/catch-syscall.exp b/gdb/testsuite/gdb.base/catch-syscall.exp
> index 0588cb35d87..d8ea466cf00 100644
> --- a/gdb/testsuite/gdb.base/catch-syscall.exp
> +++ b/gdb/testsuite/gdb.base/catch-syscall.exp
> @@ -134,13 +134,7 @@ proc check_return_from_syscall { syscall { pattern "" } } {
>   		return 1
>   	    }
>   	    -re -wrap ".*Breakpoint $decimal, main .*" {
> -		# On Powerpc the kernel does not report the returned from
> -		# syscall as expected by the test.  GDB bugzilla 28623.
> -		if { [istarget "powerpc64*-linux*"] } {
> -		    xfail $thistest
> -		} else {
> -		    fail $thistest
> -		}
> +		fail $thistest
>   		return 0
>   	    }
>   	}
> 
> base-commit: 790ce1f70c25129b35e060329cdf2915a6def9fd

This looks good to me.

-- 
John Baldwin