[LTP] [PATCH v2] syscalls/signal06: loop being clobbered by syscall on clang

[Viktor Martensson]

Hello!

The syscall will get the return value in %rax I believe, so it may be wise
to indicate that it gets modified no? I would assume something like

*int foo; *
asm volatile ("syscall" : *"=a" (foo)*: "a"(__NR_tkill), "D"(pid),
"S"(SIGHUP) : "rcx", "r11");

would instruct the compiler to expect that %rax has been modified, and it
can't rely on it staying the same. An optimization pass may otherwise take
away the repeated setting of __NR_tkill in %rax.

/V

On Fri, Aug 19, 2022 at 2:31 PM 'Edward Liaw' via kernel-team <
kernel-team@android.com> wrote:

> Indicate to the compiler that the syscall will modify registers rcx
> and r11 to prevent loop from getting clobbered.
>
> Signed-off-by: Edward Liaw <edliaw@google.com>
>
> ---
> When I combined the asm instruction into one line, it threw an error
> that the "Asm-specifier for input or output variable conflicts with asm
> clobber list" for rax.  I omitted it for now, but I'm not sure if that
> is correct.
>
> Also, is it ok to change the subject line like I did?
> ---
>  testcases/kernel/syscalls/signal/signal06.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/testcases/kernel/syscalls/signal/signal06.c
> b/testcases/kernel/syscalls/signal/signal06.c
> index 64f886ee3..fba380610 100644
> --- a/testcases/kernel/syscalls/signal/signal06.c
> +++ b/testcases/kernel/syscalls/signal/signal06.c
> @@ -72,8 +72,7 @@ void test(void)
>         while (D == VALUE && loop < LOOPS) {
>                 /* sys_tkill(pid, SIGHUP); asm to avoid save/reload
>                  * fp regs around c call */
> -               asm ("" : : "a"(__NR_tkill), "D"(pid), "S"(SIGHUP));
> -               asm ("syscall" : : : "ax");
> +               asm volatile ("syscall" : : "a"(__NR_tkill), "D"(pid),
> "S"(SIGHUP) : "rcx", "r11");
>
>                 loop++;
>         }
> --
> 2.37.2.609.g9ff673ca1a-goog
>
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