Re: [LTP] [PATCH 0/3] safe_macros: Fix undefined behaviour in vararg handling

[Petr Vorel <pvorel@suse.cz>]

Hi Tudor,

> Accessing elements in an empty va_list results in undefined behaviour[0]
> that can include accessing arbitrary stack memory. While typically this
> doesn't raise a fault, some new more security-oriented architectures
> (e.g. CHERI[1] or Morello[2]) don't allow it.

> Therefore, remove the variadicness from safe_* wrappers that always call
> the functions with the optional argument included.

> Adapt the respective SAFE_* macros to handle the change by passing a
> default argument if they're omitted.

Thanks for an interesting patchset!

I wonder if it's a correct approach as C API allows
int open(const char *pathname, int flags);
we will replace it 
int open(const char *pathname, int flags, mode_t mode);
where mode is 0.
But as it's only in safe_* it should be ok.
We still have some open() tests without mode, i.e.
testcases/kernel/syscalls/open/open06.c

Unfortunately some tests need to be adjusted, at least all tests in
testcases/kernel/syscalls/fgetxattr will fail due int-conversion,
as they use NULL as the third parameter:

$ export CFLAGS="-Werror=conversion"
$ ./configure
$ cd testcases/kernel/syscalls/fgetxattr
$ make clean
$ make V=1
gcc -I../../../../include -I../../../../include -I../../../../include/old/ -Werror=int-conversion -g -O2 -fno-strict-aliasing -pipe -Wall -W -Wold-style-definition -std=gnu99 -L../../../../lib fgetxattr01.c  -lltp -o fgetxattr01
In file included from ../../../../include/tst_test.h:98,
                 from fgetxattr01.c:34:
fgetxattr01.c: In function ‘setup’:
../../../../include/tst_safe_macros.h:90:67: error: passing argument 6 of ‘safe_open’ makes integer from pointer without a cast [-Werror=int-conversion]
   90 |         safe_open(__FILE__, __LINE__, NULL, (pathname), (oflags), (mode))
      |                                                                   ^~~~~~
      |                                                                   |
      |                                                                   void *
../../../../include/tst_safe_macros.h:93:9: note: in expansion of macro ‘__SAFE_OPEN’
   93 |         __SAFE_OPEN((pathname), (oflags), ##__VA_ARGS__, 0)
      |         ^~~~~~~~~~~
fgetxattr01.c:114:14: note: in expansion of macro ‘SAFE_OPEN’
  114 |         fd = SAFE_OPEN(FNAME, O_RDONLY, NULL);
      |              ^~~~~~~~~
In file included from ../../../../include/tst_safe_macros.h:24:
../../../../include/safe_macros_fn.h:78:22: note: expected ‘mode_t’ {aka ‘unsigned int’} but argument is of type ‘void *’
   78 |               mode_t mode);
      |               ~~~~~~~^~~~
cc1: some warnings being treated as errors
make: *** [../../../../include/mk/rules.mk:43: fgetxattr01] Error 1

Found on recent openSUSE Tumbleweed which has 2.36. Also our CI has caught it on
Fedora (which also uses 2.36):
https://github.com/pevik/ltp/actions/runs/3573215390/jobs/6007016572

Kind regards,
Petr

> [0]: [ISO/IEC 9899:2011] Programming Languages—C, 3rd ed, paragraph 7.16.1.1
> [1]: https://www.cl.cam.ac.uk/research/security/ctsrd/cheri/
> [2]: https://www.morello-project.org/

> Tudor Cretu (3):
>   safe_open: Fix undefined behaviour in vararg handling
>   safe_openat: Fix undefined behaviour in vararg handling
>   safe_semctl: Fix undefined behaviour in vararg handling

>  include/old/safe_macros.h   |  6 ++++--
>  include/safe_macros_fn.h    |  3 ++-
>  include/tst_safe_file_at.h  | 10 ++++++----
>  include/tst_safe_macros.h   |  6 ++++--
>  include/tst_safe_sysv_ipc.h | 14 +++++++++-----
>  lib/safe_macros.c           | 13 +------------
>  lib/tst_cgroup.c            |  2 +-
>  lib/tst_safe_file_at.c      | 11 +++--------
>  lib/tst_safe_sysv_ipc.c     | 10 +---------
>  9 files changed, 31 insertions(+), 44 deletions(-)

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