From mboxrd@z Thu Jan 1 00:00:00 1970
From: bugzilla-daemon-590EEB7GvNiWaY/ihj7yzEB+6BGkLq7r@public.gmane.org
Subject: [Bug 70801] ptrace PEEKDATA API is incorrect
Date: Wed, 19 Feb 2014 11:39:13 +0000
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https://bugzilla.kernel.org/show_bug.cgi?id=70801
Michael Kerrisk changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
Resolution|--- |CODE_FIX
--- Comment #3 from Michael Kerrisk ---
(In reply to Mike Frysinger from comment #1)
> it depends entirely on the arch. a bunch do as the man page describes. the
> generic ptrace layer is not used by a bunch.
>
> for example alpha/kernel/ptrace.c:
> case PTRACE_PEEKTEXT: /* read word at location addr. */
> case PTRACE_PEEKDATA:
> copied = access_process_vm(child, addr, &tmp, sizeof(tmp), 0);
> ret = -EIO;
> if (copied != sizeof(tmp))
> break;
>
> force_successful_syscall_return();
> ret = tmp;
> break;
>
> or ia64/kernel/ptrace.c:
> case PTRACE_PEEKTEXT:
> case PTRACE_PEEKDATA:
> /* read word at location addr */
> if (access_process_vm(child, addr, &data, sizeof(data), 0)
> != sizeof(data))
> return -EIO;
> /* ensure return value is not mistaken for error code */
> force_successful_syscall_return();
> return data;
>
> it's the API that strace uses:
> strace/util.c:
> u.val = ptrace(PTRACE_PEEKDATA, pid, (char *) addr, 0);
>
> the generic glibc ignores it too:
> glibc/misc/ptrace.c:
> case PTRACE_PEEKDATA:
> va_start(ap, request);
> pid = va_arg(ap, pid_t);
> addr = va_arg(ap, void *);
> va_end(ap);
> break;
>
> although apparently glibc's linux layer has been rewriting this silently:
> if (request > 0 && request < 4)
> data = &ret;
> ...
> if (res >= 0 && request > 0 && request < 4)
> {
> __set_errno (0);
> return ret;
> }
>
> where request {1,2,3} are PTRACE_PEEK{TEXT,DATA,USER}
Thanks for the above, Mike.
> as mentioned before, the man page is geared towards documenting the C
> library interface rather than the syscall one. so the current docs are
> correct. this could use noting in the NOTES section.
Yes. I've noted that point in various informal contexts (such as mailing
lists), and it's also hidden away here
https://www.kernel.org/doc/man-pages/todo.html#migrate_to_kernel_source . But,
like that info in the ptrace() page, it should be more obvious. I committed a
patch, https://www.kernel.org/doc/man-pages/todo.html#migrate_to_kernel_source
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