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

NOTE: this is from gcc, but obviously also clang has the same problem,
even without -Werror=int-conversion in CFLAGS, obviously it's the default.
That's why it was found on Fedora, which we test with clang.

Kind regards,
Petr

> 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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