From 613e601bb4b50dc359b41f162a5b629449e4bbea Mon Sep 17 00:00:00 2001
From: Carlos Santos <casantos@redhat.com>
Date: Fri, 18 Oct 2019 22:02:49 -0300
Subject: [PATCH] Circumvent bug in uClibc-ng syscall() on x86_64 systems

On uClibc at least up to v1.0.32, syscall() for x86_64 is defined in
libc/sysdeps/linux/x86_64/syscall.S as

syscall:
        movq %rdi, %rax         /* Syscall number -> rax.  */
        movq %rsi, %rdi         /* shift arg1 - arg5.  */
        movq %rdx, %rsi
        movq %rcx, %rdx
        movq %r8, %r10
        movq %r9, %r8
        movq 8(%rsp),%r9        /* arg6 is on the stack.  */
        syscall                 /* Do the system call.  */
        cmpq $-4095, %rax       /* Check %rax for error.  */
        jae __syscall_error     /* Branch forward if it failed.  */
        ret                     /* Return to caller.  */

And __syscall_error is defined in
libc/sysdeps/linux/x86_64/__syscall_error.c as

int __syscall_error(void) attribute_hidden;
int __syscall_error(void)
{
	register int err_no __asm__ ("%rcx");
	__asm__ ("mov %rax, %rcx\n\t"
	         "neg %rcx");
	__set_errno(err_no);
	return -1;
}

Notice that __syscall_error returns -1 as a 32-bit int in %rax, a 64-bit
register i.e. 0x00000000ffffffff (decimal 4294967295). When this value
is compared to -1 in _sys_chk_seccomp_flag_kernel() the result is false,
leading the function to always return 0.

Prevent the error by coercing the return value of syscall() to int in a
temporary variable before comparing it to -1. We could use just an (int)
cast but the variable makes the code more readable and the machine code
generated by the compiler is the same in both cases.

All other syscall() invocations were inspected and they either already
coerce the result to int or do not compare it to -1.

The same problem probably occurs on other 64-bit systems but so far only
x86_64 was tested.

A bug report is being submitted to uClibc.

Signed-off-by: Carlos Santos <casantos@redhat.com>
---
 src/system.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/src/system.c b/src/system.c
index 8e5aafc..811b401 100644
--- a/src/system.c
+++ b/src/system.c
@@ -215,10 +215,12 @@ static int _sys_chk_seccomp_flag_kernel(int flag)
 	/* this is an invalid seccomp(2) call because the last argument
 	 * is NULL, but depending on the errno value of EFAULT we can
 	 * guess if the filter flag is supported or not */
-	if (sys_chk_seccomp_syscall() == 1 &&
-	    syscall(_nr_seccomp, SECCOMP_SET_MODE_FILTER, flag, NULL) == -1 &&
-	    errno == EFAULT)
+	int rc;
+	if (sys_chk_seccomp_syscall() == 1) {
+	    rc = syscall(_nr_seccomp, SECCOMP_SET_MODE_FILTER, flag, NULL);
+	    if (rc == -1 && errno == EFAULT)
 		return 1;
+	}
 
 	return 0;
 }
-- 
2.18.1