qnx4: avoid stringop-overread errors

Linux Kernel / Linux Kernel - Linus Torvalds [linux-foundation.org] - 15 September 2021 20:56 UTC

The qnx4 directory entries are 64-byte blocks that have different contents depending on the a status byte that is in the last byte of the block.

In particular, a directory entry can be either a "link info" entry with a 48-byte name and pointers to the real inode information, or an "inode entry" with a smaller 16-byte name and the full inode information.

But the code was written to always just treat the directory name as if it was part of that "inode entry", and just extend the name to the longer case if the status byte said it was a link entry.

That work just fine and gives the right results, but now that gcc is tracking data structure accesses much more, the code can trigger a compiler error about using up to 48 bytes (the long name) in a structure that only has that shorter name in it:

fs/qnx4/dir.c: In function ‘qnx4_readdir’: fs/qnx4/dir.c:51:32: error: ‘strnlen’ specified bound 48 exceeds source size 16 [-Werror=stringop-overread] 51 | size = strnlen(de->di_fname, size); | ^~~~~~~~~~~~~~~~~~~~~~~~~~~ In file included from fs/qnx4/qnx4.h:3, from fs/qnx4/dir.c:16: include/uapi/linux/qnx4_fs.h:45:25: note: source object declared here 45 | char di_fname[QNX4_SHORT_NAME_MAX]; | ^~~~~~~~

which is because the source code doesn't really make this whole "one of two different types" explicit.

Fix this by introducing a very explicit union of the two types, and basically explaining to the compiler what is really going on.

b7213ffa0e58 qnx4: avoid stringop-overread errors
fs/qnx4/dir.c | 51 ++++++++++++++++++++++++++++++++++-----------------
1 file changed, 34 insertions(+), 17 deletions(-)

Upstream: git.kernel.org


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