Re: [PATCH 1/2] x86_64,signal: Save and restore SS in signal frames

[Andy Lutomirski <luto@amacapital.net>]

Hi Ingo,

Here's a blast from the past.  I asked Andi today, and he seems
conceptually okay with this.  Could you consider applying it?  Aside
from being a bug fix IMO, it will allow my sigreturn test to work
reasonably well as a native 64-bit program.

I checked: it still applies cleanly.

--Andy

On Fri, Jul 11, 2014 at 9:29 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> The comment in the signal code says that apps can save/restore other
> segments on their own.  It's true that apps can *save* SS on their
> own, but there's no way for apps to restore it: SYSCALL effectively
> resets SS to __USER_DS, so any value that user code tries to load
> into SS gets lost on entry to sigreturn.
>
> This recycles two padding bytes in the segment selector area for SS.
>
> I suspect that 64-bit programs that try to run 16-bit code and uses
> signals will have a lot of trouble without this.
>
> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> ---
>  arch/x86/include/asm/sigcontext.h      | 2 +-
>  arch/x86/include/uapi/asm/sigcontext.h | 2 +-
>  arch/x86/kernel/signal.c               | 8 +-------
>  3 files changed, 3 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/include/asm/sigcontext.h b/arch/x86/include/asm/sigcontext.h
> index 9dfce4e..f910cdc 100644
> --- a/arch/x86/include/asm/sigcontext.h
> +++ b/arch/x86/include/asm/sigcontext.h
> @@ -59,7 +59,7 @@ struct sigcontext {
>         unsigned short cs;
>         unsigned short gs;
>         unsigned short fs;
> -       unsigned short __pad0;
> +       unsigned short ss;
>         unsigned long err;
>         unsigned long trapno;
>         unsigned long oldmask;
> diff --git a/arch/x86/include/uapi/asm/sigcontext.h b/arch/x86/include/uapi/asm/sigcontext.h
> index d8b9f90..076b11f 100644
> --- a/arch/x86/include/uapi/asm/sigcontext.h
> +++ b/arch/x86/include/uapi/asm/sigcontext.h
> @@ -179,7 +179,7 @@ struct sigcontext {
>         __u16 cs;
>         __u16 gs;
>         __u16 fs;
> -       __u16 __pad0;
> +       __u16 ss;
>         __u64 err;
>         __u64 trapno;
>         __u64 oldmask;
> diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
> index 2851d63..08d7a9e 100644
> --- a/arch/x86/kernel/signal.c
> +++ b/arch/x86/kernel/signal.c
> @@ -94,15 +94,8 @@ int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc,
>                 COPY(r15);
>  #endif /* CONFIG_X86_64 */
>
> -#ifdef CONFIG_X86_32
>                 COPY_SEG_CPL3(cs);
>                 COPY_SEG_CPL3(ss);
> -#else /* !CONFIG_X86_32 */
> -               /* Kernel saves and restores only the CS segment register on signals,
> -                * which is the bare minimum needed to allow mixed 32/64-bit code.
> -                * App's signal handler can save/restore other segments if needed. */
> -               COPY_SEG_CPL3(cs);
> -#endif /* CONFIG_X86_32 */
>
>                 get_user_ex(tmpflags, &sc->flags);
>                 regs->flags = (regs->flags & ~FIX_EFLAGS) | (tmpflags & FIX_EFLAGS);
> @@ -164,6 +157,7 @@ int setup_sigcontext(struct sigcontext __user *sc, void __user *fpstate,
>                 put_user_ex(regs->cs, &sc->cs);
>                 put_user_ex(0, &sc->gs);
>                 put_user_ex(0, &sc->fs);
> +               put_user_ex(regs->ss, &sc->ss);
>  #endif /* CONFIG_X86_32 */
>
>                 put_user_ex(fpstate, &sc->fpstate);
> --
> 1.9.3
>



-- 
Andy Lutomirski
AMA Capital Management, LLC