Bug 241609

Description of problem:
Linux currently lacks a proper fexecve() system call, so fexecve() is
implemented as a glibc library call that looks up the file to execute in
/proc/self/fds and calls execve().  However, if a program that starts as root
drops root privileges by calling setuid() or seteuid() before calling fexecve()
(for instance, in order to execute a program as another user), then it no longer
has read access to /proc/self/fds, and fexecve() fails.

The man page for fexecve() does not mention this failure mode.  (Incidentally,
the man page for fexecve() in not included in FC5.)

Version-Release number of selected component (if applicable):
This bug occurs in glibc-2.4-11 from FC5.  Since sysdeps/unix/sysv/linux/fexec.c
in the glibc sources (the file implementing the fexecve() emulation) hasn't
changed, I expect that this issue occurs in all versions of glibc that implement
fexecve().

Steps to Reproduce:
Run the attached program *as root*.  (If user id 99 doesn't exist on your
system, change the arguments of the setuid() or seteuid() call a user id that
does exist.)  It should produce no output and return 0, but it instead prints
"fexecve() failed; errno = 13" and returns 255, indicating that the call to
fexecve() failed. (errno code 13 is EACCES).  If the setuid() and seteuid()
calls are both commented out, the program works as expected.

Additional Info:
I don't think that this can be fixed in libc; we need a proper kernel
implementation.  A work-around is to use execve() directly, but this leads to
race conditions under some circumstances (such as when trying to execute a file
with the file owner's permissions).