Re: PATCH: Add x32 arch_prctl support

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Mon, May 21, 2012 at 4:26 PM, Roland McGrath <roland@hack.frob.com> wrote:
>> Change it to unsigned long long int makes it impossible
>> to get address as pointer since it will override memory on stack.
>
> There is some misunderstanding. ?What I suggested would not make any
> difference to the compiled code of arch_prctl.

The goes back to user uint32_t vs kernel uint64_t:

http://sourceware.org/ml/libc-alpha/2012-05/msg01519.html

The difference is movq vs movl.  There is no need for movq
since x32 will only pass unsigned 32-bit value to kernel.

>> I am trying to limit my x32 GDB change as small as possible:
>>
>> hjl@gnu-6 gdb]$ grep PTRACE_ARCH_PRCTL ?amd64-linux-nat.c
>> #ifndef PTRACE_ARCH_PRCTL
>> #define PTRACE_ARCH_PRCTL ? ? ?30
>> ? ? ? ? if (ptrace (PTRACE_ARCH_PRCTL, lwpid, base, ARCH_GET_FS) == 0)
>> ? ? ? ? if (ptrace (PTRACE_ARCH_PRCTL, lwpid, base, ARCH_GET_GS) == 0)
>> [hjl@gnu-6 gdb]$
>>
>> Without PTRACE_ARCH_PRCTL means I have to write a whole
>> new patch to use the new interface. ? It will make it harder to enable
>> x32 in GDB.
>
> Sometimes the least change everywhere is not the cleanest answer anywhere.
>
> diff --git a/gdb/amd64-linux-nat.c b/gdb/amd64-linux-nat.c
> index 5ebba3a..aaa1e62 100644
> --- a/gdb/amd64-linux-nat.c
> +++ b/gdb/amd64-linux-nat.c
> @@ -479,11 +479,41 @@ ps_get_thread_area (const struct ps_prochandle *ph,
> ? ? ? switch (idx)
> ? ? ? ?{
> ? ? ? ?case FS:
> - ? ? ? ? if (ptrace (PTRACE_ARCH_PRCTL, lwpid, base, ARCH_GET_FS) == 0)
> + ? ? ? ? if (sizeof(unsigned long) == 4)
> + ? ? ? ? ? {
> + ? ? ? ? ? ? /* PTRACE_ARCH_PRCTL is obsolete since 2.6.25, where the
> + ? ? ? ? ? ? ? ?fs_base and gs_base fields of user_regs_struct can be
> + ? ? ? ? ? ? ? ?used directly. ?We use it only to support older
> + ? ? ? ? ? ? ? ?kernels. ?But x32 kernels do not support it, so we
> + ? ? ? ? ? ? ? ?don't try to use it. ?There were no x32 kernels so old
> + ? ? ? ? ? ? ? ?that fs_base wasn't usable. ?*/
> + ? ? ? ? ? ? unsigned long fs;
> + ? ? ? ? ? ? errno = 0;
> + ? ? ? ? ? ? fs = ptrace (PTRACE_PEEKUSR, lwpid,
> + ? ? ? ? ? ? ? ? ? ? ? ? ?offsetof (struct user_regs_struct, fs_base), 0);
> + ? ? ? ? ? ? if (errno == 0)
> + ? ? ? ? ? ? ? {
> + ? ? ? ? ? ? ? ? *base = fs;
> + ? ? ? ? ? ? ? ? return PS_OK;
> + ? ? ? ? ? ? ? }
> + ? ? ? ? ? }
> + ? ? ? ? else if (ptrace (PTRACE_ARCH_PRCTL, lwpid, base, ARCH_GET_FS) == 0)
> ? ? ? ? ? ?return PS_OK;
> ? ? ? ? ?break;
> ? ? ? ?case GS:
> - ? ? ? ? if (ptrace (PTRACE_ARCH_PRCTL, lwpid, base, ARCH_GET_GS) == 0)
> + ? ? ? ? if (sizeof(unsigned long) == 4)
> + ? ? ? ? ? {
> + ? ? ? ? ? ? unsigned long gs;
> + ? ? ? ? ? ? errno = 0;
> + ? ? ? ? ? ? gs = ptrace (PTRACE_PEEKUSR, lwpid,
> + ? ? ? ? ? ? ? ? ? ? ? ? ?offsetof (struct user_regs_struct, gs_base), 0);
> + ? ? ? ? ? ? if (errno == 0)
> + ? ? ? ? ? ? ? {
> + ? ? ? ? ? ? ? ? *base = gs;
> + ? ? ? ? ? ? ? ? return PS_OK;
> + ? ? ? ? ? ? ? }
> + ? ? ? ? ? }
> + ? ? ? ? else if (ptrace (PTRACE_ARCH_PRCTL, lwpid, base, ARCH_GET_GS) == 0)
> ? ? ? ? ? ?return PS_OK;
> ? ? ? ? ?break;
> ? ? ? ?default: ? ? ? ? ? ? ? ? ? /* Should not happen. ?*/

Thanks.  But since x32 kernel takes PTRACE_ARCH_PRCTL, I
still need x32 ptrace to either handle it properly or set EINVAL
when PTRACE_ARCH_PRCTL is passed.

-- 
H.J.