Re: [PATCH v3 10/27] linux-user/i386: Implement setup_sigtramp

[Philippe Mathieu-Daudé]

On 9/24/21 18:59, Richard Henderson wrote:
> Create and record the two signal trampolines.
> Use them when the guest does not use SA_RESTORER.
> Note that x86_64 does not use this code.
>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>   linux-user/i386/target_signal.h   |  2 ++
>   linux-user/x86_64/target_signal.h |  3 ++
>   linux-user/i386/signal.c          | 56 +++++++++++++++++++++----------
>   3 files changed, 43 insertions(+), 18 deletions(-)
>
> diff --git a/linux-user/i386/target_signal.h b/linux-user/i386/target_signal.h
> index 50361af874..64d09f2e75 100644
> --- a/linux-user/i386/target_signal.h
> +++ b/linux-user/i386/target_signal.h
> @@ -22,4 +22,6 @@ typedef struct target_sigaltstack {
>   #include "../generic/signal.h"
>   
>   #define TARGET_ARCH_HAS_SETUP_FRAME
> +#define TARGET_ARCH_HAS_SIGTRAMP_PAGE 1
> +
>   #endif /* I386_TARGET_SIGNAL_H */
> diff --git a/linux-user/x86_64/target_signal.h 
> b/linux-user/x86_64/target_signal.h
> index 4ea74f20dd..4673c5a886 100644
> --- a/linux-user/x86_64/target_signal.h
> +++ b/linux-user/x86_64/target_signal.h
> @@ -21,4 +21,7 @@ typedef struct target_sigaltstack {
>   
>   #include "../generic/signal.h"
>   
> +/* For x86_64, use of SA_RESTORER is mandatory. */
> +#define TARGET_ARCH_HAS_SIGTRAMP_PAGE 0
> +
>   #endif /* X86_64_TARGET_SIGNAL_H */
> diff --git a/linux-user/i386/signal.c b/linux-user/i386/signal.c
> index 3b4b55fc0a..94ab6396a0 100644
> --- a/linux-user/i386/signal.c
> +++ b/linux-user/i386/signal.c
> @@ -310,6 +310,22 @@ get_sigframe(struct target_sigaction *ka, CPUX86State 
> *env, size_t frame_size)
>   }
>   
>   #ifndef TARGET_X86_64
> +static void install_sigtramp(void *tramp)
> +{
> +    /* This is popl %eax ; movl $syscall,%eax ; int $0x80 */
> +    __put_user(0xb858, (uint16_t *)(tramp + 0));
> +    __put_user(TARGET_NR_sigreturn, (int *)(tramp + 2));

I know this is mostly code movement, but using uint32_t would
make it easier to read.

> +    __put_user(0x80cd, (uint16_t *)(tramp + 6));
> +}
> +
> +static void install_rt_sigtramp(void *tramp)
> +{
> +    /* This is movl $syscall,%eax ; int $0x80 */
> +    __put_user(0xb8, (char *)(tramp + 0));

Ditto uint8_t,

> +    __put_user(TARGET_NR_rt_sigreturn, (int *)(tramp + 1));

and uint32_t.

> +    __put_user(0x80cd, (uint16_t *)(tramp + 5));
> +}
> +
>   /* compare linux/arch/i386/kernel/signal.c:setup_frame() */
>   void setup_frame(int sig, struct target_sigaction *ka,
>                    target_sigset_t *set, CPUX86State *env)
> @@ -338,16 +354,9 @@ void setup_frame(int sig, struct target_sigaction *ka,
>       if (ka->sa_flags & TARGET_SA_RESTORER) {
>           __put_user(ka->sa_restorer, &frame->pretcode);
>       } else {
> -        uint16_t val16;
> -        abi_ulong retcode_addr;
> -        retcode_addr = frame_addr + offsetof(struct sigframe, retcode);
> -        __put_user(retcode_addr, &frame->pretcode);
> -        /* This is popl %eax ; movl $,%eax ; int $0x80 */
> -        val16 = 0xb858;
> -        __put_user(val16, (uint16_t *)(frame->retcode+0));
> -        __put_user(TARGET_NR_sigreturn, (int *)(frame->retcode+2));
> -        val16 = 0x80cd;
> -        __put_user(val16, (uint16_t *)(frame->retcode+6));
> +        /* This is no longer used, but is retained for ABI compatibility. */
> +        install_sigtramp(frame->retcode);
> +        __put_user(default_sigreturn, &frame->pretcode);
>       }
>   
>       /* Set up registers for signal handler */
> @@ -416,14 +425,9 @@ void setup_rt_frame(int sig, struct target_sigaction *ka,
>       if (ka->sa_flags & TARGET_SA_RESTORER) {
>           __put_user(ka->sa_restorer, &frame->pretcode);
>       } else {
> -        uint16_t val16;
> -        addr = frame_addr + offsetof(struct rt_sigframe, retcode);
> -        __put_user(addr, &frame->pretcode);
> -        /* This is movl $,%eax ; int $0x80 */
> -        __put_user(0xb8, (char *)(frame->retcode+0));
> -        __put_user(TARGET_NR_rt_sigreturn, (int *)(frame->retcode+1));
> -        val16 = 0x80cd;
> -        __put_user(val16, (uint16_t *)(frame->retcode+5));
> +        /* This is no longer used, but is retained for ABI compatibility. */
> +        install_rt_sigtramp(frame->retcode);
> +        __put_user(default_rt_sigreturn, &frame->pretcode);
>       }
>   #else
>       /* XXX: Would be slightly better to return -EFAULT here if test fails
> @@ -592,3 +596,19 @@ badframe:
>       force_sig(TARGET_SIGSEGV);
>       return -TARGET_QEMU_ESIGRETURN;
>   }
> +
> +#ifndef TARGET_X86_64
> +void setup_sigtramp(abi_ulong sigtramp_page)
> +{
> +    uint16_t *tramp = lock_user(VERIFY_WRITE, sigtramp_page, 2 * 8, 0);

Shouldn't this be 8 + 7?

> +    assert(tramp != NULL);
> +
> +    default_sigreturn = sigtramp_page;
> +    install_sigtramp(tramp);
> +
> +    default_rt_sigreturn = sigtramp_page + 8;
> +    install_rt_sigtramp(tramp + 8);
> +
> +    unlock_user(tramp, sigtramp_page, 2 * 8);
> +}
> +#endif