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SubjectRe: [PATCH 2/2] x86: ia32_signal: remove unnecessary padding
Ingo Molnar writes:
>
> * Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com> wrote:
>
> > From: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
> >
> > Impact: cleanup
> >
> > Remove unnecessary paddings, this saves 4 bytes.
> >
> > Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
> > ---
> > arch/x86/ia32/ia32_signal.c | 5 +----
> > 1 files changed, 1 insertions(+), 4 deletions(-)
>
> applied your patches to tip/x86/signal:
>
> 9cc3c49: x86: ia32_signal: remove unnecessary padding
> 4a61204: x86: signal_32: introduce retcode and rt_retcode
>
> thanks!
>
> A question - this change:
>
> > @@ -427,12 +427,10 @@ int ia32_setup_frame(int sig, struct k_sigaction *ka,
> > u16 poplmovl;
> > u32 val;
> > u16 int80;
> > - u16 pad;
> > } __attribute__((packed)) code = {
> > 0xb858, /* popl %eax ; movl $...,%eax */
> > __NR_ia32_sigreturn,
> > 0x80cd, /* int $0x80 */
> > - 0,
> > };
> >
> > frame = get_sigframe(ka, regs, sizeof(*frame), &fpstate);
> > @@ -508,8 +506,7 @@ int ia32_setup_rt_frame(int sig, struct k_sigaction *ka, siginfo_t *info,
> > u8 movl;
> > u32 val;
> > u16 int80;
> > - u16 pad;
> > - u8 pad2;
> > + u8 pad;
> > } __attribute__((packed)) code = {
> > 0xb8,
> > __NR_ia32_rt_sigreturn,
>
> does not impact any ABI, because it's only about the signal trampoline
> the kernel pushes to the user-space stack - not about any
> userspace-visible signal frame detail, right?

As long as the 'char retcode[8]' field isn't reduced in size
(you can _never_ do that!), and the initial instruction
sequence up to the 'int $0x80' isn't changed (you can't change
that either), I believe this is safe.

It does cause each signal delivery to leak 2 uninitialised
kernel bytes to the end of retcode[], which seems unnecessary.


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