Re: [PATCH] hurd: libgcc unwinding support over signal trampolines

[Thomas Schwinge]

Hi Samuel!

On 2020-05-29T13:46:50+0200, Samuel Thibault <samuel.thibault@gnu.org> wrote:
> libgcc is currently missing the support for unwinding over signal
> trampolines on GNU/Hurd.

ACK.  Has been on my long TODO list for a long time.  ;-)

> The attached patch implements it.

Thanks!

I'm not intimately familiar with the unwinding implementation, but your
changes look quite what I'd have guessed they'd look like, and also
somewhat similar to 'libgcc/config/i386/linux-unwind.h'.  (I'll get
there, eventually, again..., but) I'm not currently set up to test this,
but I'll assume you have.

Which GCC branches would you like this on?

> --- /dev/null
> +++ b/src/libgcc/config/i386/gnu-unwind.h
> @@ -0,0 +1,107 @@

> + * i386 sigtramp frame we are looking for follows.
> + * (see glibc/sysdeps/mach/hurd/i386/trampoline.c assembly)
> + *
> + * rpc_wait_trampoline:
> + *   0:      b8 e7 ff ff ff          mov    $-25,%eax       mach_msg_trap
> + *   5:      9a 00 00 00 00 07 00    lcall  $7,$0
> + *  12:      89 01                   movl   %eax, (%ecx)
> + *  14:      89 dc                   movl   %ebx, %esp      switch to signal 
> stack
> + *
> + * trampoline:
> + *  16:      ff d2                   call   *%edx           call the handler 
> function
> + * RA HERE
> + *  18:      83 c4 0c                addl   $12, %esp       pop its args
> + *  21:      c3                      ret                    return to 
> sigreturn
> + *
> + * firewall:
> + *  22:      f4                      hlt
> + */
> +
> +  if (!(   *(unsigned int   *)(context->ra     ) == 0xc30cc483
> +        && *(unsigned char  *)(context->ra +  4) ==       0xf4
> +
> +        && *(unsigned int   *)(context->ra -  4) == 0xd2ffdc89
> +        && *(unsigned int   *)(context->ra -  8) == 0x01890007
> +        && *(unsigned int   *)(context->ra - 12) == 0x00000000
> +        && *(unsigned int   *)(context->ra - 16) == 0x9affffff
> +        && *(unsigned short *)(context->ra - 18) ==     0xe7b8))
> +    return _URC_END_OF_STACK;

Once we've got this in GCC, please then also cross-reference GCC's
'libgcc/config/i386/gnu-unwind.h' file in glibc's
'sysdeps/mach/hurd/i386/trampoline.c' file.


Grüße
 Thomas
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