diff mbox series

elf: Fix marking root dir as nonexist in open_path

Message ID ME3P282MB39685DC6FC209A91F072BA3CD3719@ME3P282MB3968.AUSP282.PROD.OUTLOOK.COM
State Changes Requested
Headers
Series elf: Fix marking root dir as nonexist in open_path |

Checks

Context Check Description
dj/TryBot-apply_patch success Patch applied to master at the time it was sent
dj/TryBot-32bit success Build for i686

Commit Message

Moody Liu May 8, 2023, 3:45 p.m. UTC 
  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.

Suggested-by: Qixing ksyx Xue <qixingxue@outlook.com>
---
 elf/dl-load.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

Comments

Carlos O'Donell May 8, 2023, 8:04 p.m. UTC | #1 
On 5/8/23 11:45, Moody Liu via Libc-alpha wrote:
> 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.

Thanks for working on the patch.

Could you please file a bug in bugzilla for this?

> 
> 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 '/'.

Are you able to write a containerized test case for this?

We have tests-container testing for just such "/" scenarios with a distinct
mount namespace for the tests.

>   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.
> 
> Suggested-by: Qixing ksyx Xue <qixingxue@outlook.com>
> ---
>  elf/dl-load.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/elf/dl-load.c b/elf/dl-load.c
> index fcb39a78d4..10757dd5a5 100644
> --- a/elf/dl-load.c
> +++ b/elf/dl-load.c
> @@ -1865,7 +1865,11 @@ open_path (const char *name, size_t namelen, int mode,
>  		     test whether there is any directory at all.  */
>  		  struct __stat64_t64 st;
>  
> -		  buf[buflen - namelen - 1] = '\0';
> +		  /* We only have absolute paths go into this branch.
> +		     In the rare case where 'this_dir' is only a '/', we
> +		     must keep it.  */
> +		  buflen = MAX(buflen - namelen - 1, 1);
> +		  buf[buflen] = '\0';

>  
>  		  if (__stat64_time64 (buf, &st) != 0
>  		      || ! S_ISDIR (st.st_mode))
diff mbox series

Patch

diff --git a/elf/dl-load.c b/elf/dl-load.c
index fcb39a78d4..10757dd5a5 100644
--- a/elf/dl-load.c
+++ b/elf/dl-load.c
@@ -1865,7 +1865,11 @@  open_path (const char *name, size_t namelen, int mode,
 		     test whether there is any directory at all.  */
 		  struct __stat64_t64 st;
 
-		  buf[buflen - namelen - 1] = '\0';
+		  /* We only have absolute paths go into this branch.
+		     In the rare case where 'this_dir' is only a '/', we
+		     must keep it.  */
+		  buflen = MAX(buflen - namelen - 1, 1);
+		  buf[buflen] = '\0';
 
 		  if (__stat64_time64 (buf, &st) != 0
 		      || ! S_ISDIR (st.st_mode))