On 6/22/18 5:44 PM, Jann Horn wrote:
> ptrace() with requests PTRACE_PEEKTEXT, PTRACE_PEEKDATA and
> PTRACE_PEEKUSER can set errno to zero. AFAICS this is for a good reason (so
> that you can tell the difference between a successful PEEK with a result of
> -1 and a failed PEEK, even if you forget to clear errno yourself), but it
> technically violates the rules described in the errno.3 manpage.
Hello Jann.
Thanks. Patch applied. Thanks for the very complete commit message!
Cheers,
Michael
>
> glibc snippet from sysdeps/unix/sysv/linux/ptrace.c:
>
> res = INLINE_SYSCALL (ptrace, 4, request, pid, addr, data);
> if (res >= 0 && request > 0 && request < 4)
> {
> __set_errno (0);
> return ret;
> }
>
> reproducer:
>
>
> $ cat ptrace_test.c
> #define _GNU_SOURCE
> #include <stdio.h>
> #include <err.h>
> #include <sys/wait.h>
> #include <sys/ptrace.h>
> #include <unistd.h>
> #include <errno.h>
> #include <sys/prctl.h>
> #include <signal.h>
> char foobar_data[4] = "ABCD";
> int main(void) {
> pid_t child = fork();
> if (child == -1) err(1, "fork");
> if (child == 0) {
> if (prctl(PR_SET_PDEATHSIG, SIGKILL)) err(1, "prctl");
> while (1) sleep(1);
> }
> int status;
> if (ptrace(PTRACE_ATTACH, child, NULL, NULL)) err(1, "attach");
> if (waitpid(child, &status, 0) != child) err(1, "wait");
> errno = EINVAL;
> unsigned int res = ptrace(PTRACE_PEEKDATA, child, foobar_data, NULL);
> printf("errno after PEEKDATA: %d\n", errno);
> printf("PEEKDATA result: 0x%x\n", res);
> }
> $ gcc -o ptrace_test ptrace_test.c -Wall
> $ ./ptrace_test
> errno after PEEKDATA: 0
> PEEKDATA result: 0x44434241
>
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> ---
> man2/ptrace.2 | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/man2/ptrace.2 b/man2/ptrace.2
> index aea63d2d6..53d75c570 100644
> --- a/man2/ptrace.2
> +++ b/man2/ptrace.2
> @@ -2777,6 +2777,12 @@ again.
> errors may behave in an unintended way upon an
> .BR strace (1)
> attach.)
> +.PP
> +Contrary to the normal rules, the glibc wrapper for
> +.BR ptrace ()
> +can set
> +.I errno
> +to zero.
> .SH SEE ALSO
> .BR gdb (1),
> .BR ltrace (1),
>
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
