Hi Carlos, unixmania@gmail.com wrote,
From: Carlos Santos unixmania@gmail.com
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.
Tested on all supported architectures, no regression in the testsuite seen.
Thx, applied and pushed, Waldemar