linux-kernel - setgroups32 syscall changes RCX register

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Date:	Wed, 9 Jun 2010 14:57:03 +0400
From:	"Pavel N. Iakovenko" <yak@...ras.ru>
To:	<linux-kernel@...r.kernel.org>
Subject: setgroups32 syscall changes RCX register

Hello

I noticed that setgroups32 syscall changes the value of RCX register. In my
case  - it is increased by PAGE_SIZE. Is it an acceptable behavior?
Kernel version is 2.6.31.6. Testing program is enclosed.

Please CC me on reply.

Best regards,
Pavel Iakovenko

#include <unistd.h>
#include <sys/types.h>
#include <syscall.h>
#include <grp.h>
#include <stdio.h>
#include <errno.h>

int main()
{
            gid_t groups;
            unsigned long rcx;
            int r ;

            r = initgroups("root", groups);
            if (r) {
                printf("initgroups failed\n");
                goto out;
            }
            
            printf("before setgroups32: RCX = 0x%lx\n", (unsigned
long)&groups);

            asm
            (
                        "int $0x80"
                        : "=a" (r), "=c" (rcx)
                        : "a" (SYS_setgroups32), "b"(1), "c" (&groups)
            );

            if (r) {
                printf("setgroups32 failed\n");
                goto out;
            }
            
            printf("after setgroups32: RCX = 0x%lx\n", rcx);

out:
            return 0;
}
 

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