Re: [LTP] [PATCH v1] syscalls/signal06: add volatile to loop variable

[Cyril Hrubis <chrubis@suse.cz>]

Hi!
> > It's problem with the inline assembly in the body of the while loop, the
> > call to the syscall changes the register value that is used for the D
> > variable in the case of clang, so the loop exits prematurely. We have to
> > add cx register to the clobber list for that asm statement so that
> > compiler knows that it's changed by the assembly.
> > 
> > Interfacing assembly with C is a bit tricky since you have to explain
> > to compiler which registers are changed from the assembly otherwise the
> > results are undefined.
> > 
> > The patch should look like:
> > 
> > diff --git a/testcases/kernel/syscalls/signal/signal06.c b/testcases/kernel/syscalls/signal/signal06.c
> > index 64f886ee3..78efd0fb9 100644
> > --- a/testcases/kernel/syscalls/signal/signal06.c
> > +++ b/testcases/kernel/syscalls/signal/signal06.c
> > @@ -73,7 +73,7 @@ void test(void)
> >                 /* 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 ("syscall" : : : "ax", "cx");
> Why is this even split up into two asm instructions?
> I guess there is nothing, that prevents the compiler from reordering the
> asm instructions, because it does not know, that they have side effects
> (they are not marked volatile).
> 
> asm volatile ("syscall" : : "a"(__NR_tkill), "D"(pid), "S"(SIGHUP):
> "rax", "rcx", "r11");
> 
> 
> I am not sure if there is any good reason, to split this up into two asm
> instructions and if there is a good reason, to use the short names of
> the registers.

I was wondering too, I guess it's a direct copy of a reproducer from a
kernel commit see:

https://lore.kernel.org/lkml/1414436240-13879-8-git-send-email-kamal@canonical.com/

-- 
Cyril Hrubis
chrubis@suse.cz

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