Re: [PATCH v2 1/3] [gdb/tdep] Add syscall number cache

[John Baldwin <jhb@FreeBSD.org>]

On 11/27/23 12:20 PM, 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

All 3 of these LGTM modulo one nit below.

Reviewed-by: John Baldwin <jhb@FreeBSD.org>

> ---
>   gdb/linux-nat.c                          | 53 ++++++++++++++++++++++--
>   gdb/linux-nat.h                          |  3 ++
>   gdb/testsuite/gdb.base/catch-syscall.exp |  8 +---
>   3 files changed, 53 insertions(+), 11 deletions(-)
> 
> diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
> index 7b0562cf89b..89c4622160a 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.  */
> +    }

ICYMI, Simon just changed catch_syscall_enabled to return a bool a couple of
hours before your mail.

-- 
John Baldwin