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Message-ID: <d48f66cf-9843-1575-bcf0-5117a5527004@igalia.com>
Date: Mon, 1 Sep 2025 10:58:17 +0530
From: Bhupesh Sharma <bhsharma@...lia.com>
To: Kees Cook <kees@...nel.org>, Bhupesh <bhupesh@...lia.com>
Cc: akpm@...ux-foundation.org, kernel-dev@...lia.com,
linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
linux-perf-users@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-mm@...ck.org, oliver.sang@...el.com, lkp@...el.com,
laoar.shao@...il.com, pmladek@...e.com, rostedt@...dmis.org,
mathieu.desnoyers@...icios.com, arnaldo.melo@...il.com,
alexei.starovoitov@...il.com, andrii.nakryiko@...il.com,
mirq-linux@...e.qmqm.pl, peterz@...radead.org, willy@...radead.org,
david@...hat.com, viro@...iv.linux.org.uk, ebiederm@...ssion.com,
brauner@...nel.org, jack@...e.cz, mingo@...hat.com, juri.lelli@...hat.com,
bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com,
linux-trace-kernel@...r.kernel.org, torvalds@...ux-foundation.org
Subject: Re: [PATCH v8 4/5] treewide: Switch memcpy() users of 'task->comm' to
a more safer implementation
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=TALm65ReN7YjrMw@mail.gmail.com/ #1
>>
>> Signed-off-by: Bhupesh<bhupesh@...lia.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.
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?
>
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