[patch 0/3] [RFC] kernel/glibc mismatch of "readlink" syscall?

From: Daniel Gollub
Date: Thu Oct 23 2008 - 11:06:06 EST


found something which looks for me like a kernel/glibc syscall mismatch. At
least the parameter list of "readlink" is different in the kernel compared to
glibc, POSIX and linux-man-pages. I'm not quite sure if this difference was
intended or not ...

man 3p readlink:
ssize_t readlink(const char *restrict path, char *restrict buf, size_t bufsize);

size_t readlink(const char *restrict path, char *restrict buf, size_t bufsize);

glibc (/usr/include/unistd.h):
size_t readlink (__const char *__restrict __path, char *__restrict __buf, size_t

man 2 readlink:
ssize_t readlink(const char *path, char *buf, size_t bufsiz);
asmlinkage long sys_readlink(const char __user *path, char __user *buf, int
bufsiz); ^^^

All readlink prototypes, expect the one in the kernel, have an unsigned
buffer size. Even the readlink(2) man-page, which also describes an error
statement like this:

EINVAL bufsiz is not positive.

Note: the same man-page defined bufsiz as type of size_t (unsigned).

While reviewing LTP i discovered that the "readlink03" syscall test contains a
testcase to do a functional error-path test for "EINVAL bufsiz is not positive".
This testcase is using the glibc readlink() interface, which cause a unsigned
cast of the value "-1" and let the testcase fail (actually due to gcc/glibc
fortify checks and cause a __chk_fail()).

Before workarounding the testcase, or not applying -D_FORTIFY_SOURCE=2 on LTP
build, i try to understand if there is any reason for this mismatch between
kernel and glibc/POSIX. Regarding the man-page, i'm quite certain this was a
copy&paste-error by coping the prototype from the POSIX man-page.

Even sys_readlinkat(), which got introduced a long time after sys_readlink(),
got a signed buffer size. Intended?

In the rare case all this was unintended, find patches for kernel, man-pages
and LTP to change the kernel readlink syscall interface to a unsigned buffer


best regards,
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