elf: Add test for locating libraries in root dir (bug 30435)

When dlopen is being called, efforts have been made to improve
future lookup performance. This includes marking a search path
as non-existent using `stat`. However, if the root directory
is given as a search path, there exists a bug which erroneously
marks it as non-existing.

The bug is reproduced under the following sequence:

  1. dlopen is called to open a shared library, with at least:
     1) a dependency 'A.so' not directly under the '/' directory
        (e.g. /lib/A.so), and
     2) another dependency 'B.so' resides in '/'.
  2. for this bug to reproduce, 'A.so' should be searched *before* 'B.so'.
  3. it first tries to find 'A.so' in /, (e.g. /A.so):
     - this will (obviously) fail,
     - since it's the first time we have seen the '/' directory,
       its 'status' is 'unknown'.
  4. `buf[buflen - namelen - 1] = '\0'` is executed:
     - it intends to remove the leaf and its final slash,
     - because of the speciality of '/', its buflen == namelen + 1,
     - it erroneously clears the entire buffer.
  6. it then calls 'stat' with the empty buffer:
     - which will result in an error.
  7. so it marks '/' as 'nonexisting', future lookups will not consider
     this path.
  8. while /B.so *does* exist, failure to look it up in the '/'
     directory leads to a 'cannot open shared object file' error.

This patch fixes the bug by preventing 'buflen', an index to put '\0',
from being set to 0, so that the root '/' is always kept.
Relative search paths are always considered as 'existing' so this
wont be affected.

Writeup by Moody Liu <mooodyhunter@outlook.com>

Suggested-by: Carlos O'Donell <carlos@redhat.com>
Signed-off-by: Qixing ksyx Xue <qixingxue@outlook.com>
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/elf/Makefile b/elf/Makefile
index e262f3e6b1..8640138a72 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -325,6 +325,7 @@ static-dlopen-environment = \
   LD_LIBRARY_PATH=$(ld-library-path):$(common-objpfx)dlfcn
 tst-tls9-static-ENV = $(static-dlopen-environment)
 tst-single_threaded-static-dlopen-ENV = $(static-dlopen-environment)
+tst-rootdir-ENV = LD_LIBRARY_PATH=/
 
 tests += \
   argv0test \
@@ -506,6 +507,7 @@ tests-container += \
   tst-dlopen-tlsmodid-container \
   tst-pldd \
   tst-preload-pthread-libc \
+  tst-rootdir \
   # tests-container
 
 test-srcs = \
@@ -855,6 +857,7 @@ modules-names += \
   tst-relsort1mod1 \
   tst-relsort1mod2 \
   tst-ro-dynamic-mod \
+  tst-rootdir-lib \
   tst-single_threaded-mod1 \
   tst-single_threaded-mod2 \
   tst-single_threaded-mod3 \