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

[Cyril Hrubis <chrubis@suse.cz>]

Hi!
> Thanks! The bug was closed as 'adding "cx" worked'. Reading topperc's comment it
> looks like there no other way to fix the issue on clang than workaround with
> volatile. Does it mean that it's a syscall problem and clang can do nothing
> about it?

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");

                loop++;
        }

Although it may not be a complete as the llwm issue suggests we should
have a look at calling conventions for the syscall and check if we need
to add any other registers to the clobber list.

-- 
Cyril Hrubis
chrubis@suse.cz

-- 
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