Using {memcpy,strncpy,strcpy,kstrdup} to copy the task comm relies on the
length of task comm. Changes in the task comm could result in a destination
string that is overflow. Therefore, we should explicitly ensure the destination
string is always NUL-terminated, regardless of the task comm. This approach
will facilitate future extensions to the task comm.

As suggested by Linus [0], we can identify all relevant code with the
following git grep command:

  git grep 'memcpy.*->comm\>'
  git grep 'kstrdup.*->comm\>'
  git grep 'strncpy.*->comm\>'
  git grep 'strcpy.*->comm\>'

PATCH #2~#4:   memcpy
PATCH #5~#6:   kstrdup
PATCH #7:      strncpy
PATCH #8~#9:   strcpy

There is a BUILD_BUG_ON() inside get_task_comm(), so when you use
get_task_comm(), it implies that the BUILD_BUG_ON() is necessary. However,
we don't want to impose this restriction on code where the length can be
changed, so we use __get_task_comm(), rather than the get_task_comm().

One use case of get_task_comm() is in code that has already exposed the
length to userspace. In such cases, we specifically add the BUILD_BUG_ON()
to prevent developers from changing it. For more information, see
commit 95af469c4f60 ("fs/binfmt_elf: replace open-coded string copy with
get_task_comm").

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/all/CAHk-=wjAmmHUg6vho1KjzQi2=psR30+CogFd4aXrThr2gsiS4g@mail.gmail.com/ (local) [0]

Changes:
v4->v5:
- Drop changes in the mm/kmemleak.c as it was fixed by
  commit 0b84780134fb ("mm/kmemleak: replace strncpy() with strscpy()")
- Drop changes in kernel/tsacct.c as it was fixed by
  commmit 0fe2356434e ("tsacct: replace strncpy() with strscpy()")

v3->v4: https://lore.kernel.org/linux-mm/20240729023719.1933-1-laoar.shao@gmail.com/ (local)
- Rename __kstrndup() to __kmemdup_nul() and define it inside mm/util.c
  (Matthew)
- Remove unused local varaible (Simon)

v2->v3: https://lore.kernel.org/all/20240621022959.9124-1-laoar.shao@gmail.com/ (local)
- Deduplicate code around kstrdup (Andrew)
- Add commit log for dropping task_lock (Catalin)

v1->v2: https://lore.kernel.org/bpf/20240613023044.45873-1-laoar.shao@gmail.com/ (local)
- Add comment for dropping task_lock() in __get_task_comm() (Alexei)
- Drop changes in trace event (Steven)
- Fix comment on task comm (Matus)

v1: https://lore.kernel.org/all/20240602023754.25443-1-laoar.shao@gmail.com/ (local)

Yafang Shao (9):
  fs/exec: Drop task_lock() inside __get_task_comm()
  auditsc: Replace memcpy() with __get_task_comm()
  security: Replace memcpy() with __get_task_comm()
  bpftool: Ensure task comm is always NUL-terminated
  mm/util: Fix possible race condition in kstrdup()
  mm/util: Deduplicate code in {kstrdup,kstrndup,kmemdup_nul}
  tracing: Replace strncpy() with __get_task_comm()
  net: Replace strcpy() with __get_task_comm()
  drm: Replace strcpy() with __get_task_comm()

 drivers/gpu/drm/drm_framebuffer.c     |  2 +-
 drivers/gpu/drm/i915/i915_gpu_error.c |  2 +-
 fs/exec.c                             | 10 ++++-
 include/linux/sched.h                 |  4 +-
 kernel/auditsc.c                      |  6 +--
 kernel/trace/trace.c                  |  2 +-
 kernel/trace/trace_events_hist.c      |  2 +-
 mm/util.c                             | 61 ++++++++++++---------------
 net/ipv6/ndisc.c                      |  2 +-
 security/lsm_audit.c                  |  4 +-
 security/selinux/selinuxfs.c          |  2 +-
 tools/bpf/bpftool/pids.c              |  2 +
 12 files changed, 49 insertions(+), 50 deletions(-)

-- 
2.34.1