On Mon, 1 Sep 2025 10:58:17 +0530
Bhupesh Sharma <bhsha...@igalia.com> wrote:
> Hi Kees,
>
> On 8/25/25 7:31 PM, Kees Cook wrote:
> > On Thu, Aug 21, 2025 at 03:51:51PM +0530, Bhupesh wrote:
> >> As Linus mentioned in [1], currently we have several memcpy() use-cases
> >> which use 'current->comm' to copy the task name over to local copies.
> >> For an example:
> >>
> >> ...
> >> char comm[TASK_COMM_LEN];
> >> memcpy(comm, current->comm, TASK_COMM_LEN);
> >> ...
> >>
> >> These should be rather calling a wrappper like "get_task_array()",
> >> which is implemented as:
> >>
> >> static __always_inline void
> >> __cstr_array_copy(char *dst,
> >> const char *src, __kernel_size_t size)
> >> {
> >> memcpy(dst, src, size);
> >> dst[size] = 0;
> >> }
> >>
> >> #define get_task_array(dst,src) \
> >> __cstr_array_copy(dst, src, __must_be_array(dst))
> >>
> >> The relevant 'memcpy()' users were identified using the following search
> >> pattern:
> >> $ git grep 'memcpy.*->comm\>'
> >>
> >> Link:https://lore.kernel.org/all/CAHk-=wi5c=_-FBGo_88CowJd_F-Gi6Ud9d=talm65ren7yj...@mail.gmail.com/
> >> #1
> >>
> >> Signed-off-by: Bhupesh<bhup...@igalia.com>
> >> ---
> >> include/linux/coredump.h | 2 +-
> >> include/linux/sched.h | 32 +++++++++++++++++++
> >> include/linux/tracepoint.h | 4 +--
> >> include/trace/events/block.h | 10 +++---
> >> include/trace/events/oom.h | 2 +-
> >> include/trace/events/osnoise.h | 2 +-
> >> include/trace/events/sched.h | 13 ++++----
> >> include/trace/events/signal.h | 2 +-
> >> include/trace/events/task.h | 4 +--
> >> tools/bpf/bpftool/pids.c | 6 ++--
> >> .../bpf/test_kmods/bpf_testmod-events.h | 2 +-
> >> 11 files changed, 54 insertions(+), 25 deletions(-)
> >>
> >> diff --git a/include/linux/coredump.h b/include/linux/coredump.h
> >> index 68861da4cf7c..bcee0afc5eaf 100644
> >> --- a/include/linux/coredump.h
> >> +++ b/include/linux/coredump.h
> >> @@ -54,7 +54,7 @@ extern void vfs_coredump(const kernel_siginfo_t
> >> *siginfo);
> >> do { \
> >> char comm[TASK_COMM_LEN]; \
> >> /* This will always be NUL terminated. */ \
> >> - memcpy(comm, current->comm, sizeof(comm)); \
> >> + get_task_array(comm, current->comm); \
> >> printk_ratelimited(Level "coredump: %d(%*pE): " Format "\n",
> >> \
> >> task_tgid_vnr(current), (int)strlen(comm), comm,
> >> ##__VA_ARGS__); \
> >> } while (0) \
> >> diff --git a/include/linux/sched.h b/include/linux/sched.h
> >> index 5a58c1270474..d26d1dfb9904 100644
> >> --- a/include/linux/sched.h
> >> +++ b/include/linux/sched.h
> >> @@ -1960,12 +1960,44 @@ extern void wake_up_new_task(struct task_struct
> >> *tsk);
> >>
> >> extern void kick_process(struct task_struct *tsk);
> >>
> >> +/*
> >> + * - Why not use task_lock()?
> >> + * User space can randomly change their names anyway, so locking for
> >> readers
> >> + * doesn't make sense. For writers, locking is probably necessary, as a
> >> race
> >> + * condition could lead to long-term mixed results.
> >> + * The logic inside __set_task_comm() should ensure that the task comm
> >> is
> >> + * always NUL-terminated and zero-padded. Therefore the race condition
> >> between
> >> + * reader and writer is not an issue.
> >> + */
> >> +
> >> extern void __set_task_comm(struct task_struct *tsk, const char *from,
> >> bool exec);
> >> #define set_task_comm(tsk, from) ({ \
> >> BUILD_BUG_ON(sizeof(from) < TASK_COMM_LEN); \
> >> __set_task_comm(tsk, from, false); \
> >> })
> >>
> >> +/*
> >> + * 'get_task_array' can be 'data-racy' in the destination and
> >> + * should not be used for cases where a 'stable NUL at the end'
> >> + * is needed. Its better to use strscpy and friends for such
> >> + * use-cases.
> >> + *
> >> + * It is suited mainly for a 'just copy comm to a constant-sized
> >> + * array' case - especially in performance sensitive use-cases,
> >> + * like tracing.
> >> + */
> >> +
> >> +static __always_inline void
> >> + __cstr_array_copy(char *dst, const char *src,
> >> + __kernel_size_t size)
> >> +{
> >> + memcpy(dst, src, size);
> >> + dst[size] = 0;
> >> +}
> > Please don't reinvent the wheel. :) We already have memtostr, please use
> > that (or memtostr_pad).
>
> Sure, but wouldn't we get a static assertion failure: "must be array"
> for memtostr() usage, because of the following:
>
> #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) +
> __must_be_array(arr))
>
> I think it would be easier just to set:
>
> memcpy(dst, src, size);
> dst[size -1] = 0;
>
> instead as Linus and Steven suggested.
The compiler is still likely to make a mess of it.
You really want:
*(u64 *)dst = *(u64 *)src;
*(u64 *)(dst + 8) = *(u64 *)(src + 8) & ~htobe64(0xff);
That may need something to force 8 byte alignment.
Or force 4 byte alignment and use a u64 type with 4 byte alignment.
David
>
> Thanks,
> Bhupesh
>
> >> +
> >> +#define get_task_array(dst, src) \
> >> + __cstr_array_copy(dst, src, __must_be_array(dst))
> > Uh, __must_be_array(dst) returns 0 on success. :P Are you sure you
> > tested this?
> >
>
>
