#3 [RFC] Make __syscall_error return long, as expected by syscall() callers

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.  */

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;
}

if (syscall(number, ...) == -1)
    foo();

if ((int) syscall(number, ...) == -1)
    foo();

The return type of syscall() is long so __syscall_error, which is jumped to by syscall handlers to stash an error number into errno, must return long too otherwhise it returs 4294967295L instead of -1L. For example, 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. */ In libc/sysdeps/linux/x86_64/__syscall_error.c, __syscall_error is defined 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; } So __syscall_error returns -1 as a 32-bit int in a 64-bit register, %rax (0x00000000ffffffff, whose decimal value is decimal 4294967295) and a test like this always returns false: if (syscall(number, ...) == -1) foo(); Fix the error by making __syscall_error return a long, like syscall(). The problem can be circumvented by the caller by coercing the returned value to int before comparing it to -1: if ((int) syscall(number, ...) == -1) foo(); The same problem probably occurs on other 64-bit systems but so far only x86_64 was tested, so this change must be considered experimental. Signed-off-by: Carlos Santos <unixmania@gmail.com>