getcontext() on ppc32 destroys saved parameter 1 in caller's frame

[Jakub Bogusz]

GLIBC_2.3.4 getcontext() on ppc32 changes the value of the first
parameter passed to caller's function.
Testcase:

$ cat tst_getcontext.c
#include <ucontext.h>

void test(volatile int a, volatile int b)
{
        ucontext_t uc;
        printf("before: %d %d\n", a, b);
        getcontext(&uc);
        printf("after : %d %d\n", a, b);
}

int main()
{
        test(1, 2);
}
$ ./tst_getcontext
before: 1 2
after : 2147478448 2

("volatile" is not needed when tst_getcontext is compiled without any
optimization)

It's caused by the first instruction of __getcontext()
(sysdeps/unix/sysv/linux/powerpc/powerpc32/getcontext.S:29):

ENTRY(__getcontext)
        stw     r3,_FRAME_PARM_SAVE1(r1)

__getcontext doesn't have its own stack frame, so it uses caller's
stack frame, thus destroying it...

The same applies to GLIBC_2.3.4 swapcontext(), but it may be less/not(?)
harmful as successful swapcontext() never returns.


-- 
Jakub Bogusz    http://cyber.cs.net.pl/~qboosh/
PLD Team        http://www.pld-linux.org/