[PATCH v4 3/3] exec: Add support for 64 byte 'tsk->comm_ext'

Bhupesh
2025-05-21T06:24:11Z

Historically due to the 16-byte length of TASK_COMM_LEN, the
users of 'tsk->comm' are restricted to use a fixed-size target
buffer also of TASK_COMM_LEN for 'memcpy()' like use-cases.

To fix the same, Linus suggested in [1] that we can add the
following union inside 'task_struct':
union {
char comm[TASK_COMM_LEN];
char comm_ext[TASK_COMM_EXT_LEN];
};

and then modify '__set_task_comm()' to pass 'tsk->comm_ext'
to the existing users.

This would mean that:
(1) The old common pattern of just printing with '%s' and tsk->comm
would just continue to work (as it is):

pr_alert("BUG: Bad page state in process %s pfn:%05lx\n",
current->comm, page_to_pfn(page));

(2) And, the memcpy() users of 'tsk->comm' would need to be made more
stable by ensuring that the destination buffer always has a closing
NUL character (done already in the preceding patch in this series).

So, eventually:
- users who want the existing 'TASK_COMM_LEN' behavior will get it
(existing ABIs would continue to work),
- users who just print out 'tsk->comm' as a string will get the longer
new "extended comm",
- users who do 'sizeof(->comm)' will continue to get the old value
because of the union.

[1]. https://lore.kernel.org/all/CAHk-=wjAmmHUg6vho1KjzQi2=psR30+CogFd4aXrThr2gsiS4g@mail.gmail.com

Signed-off-by: Bhupesh <bhupesh@igalia.com>
---
fs/exec.c | 6 +++---
include/linux/sched.h | 8 ++++++--
2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 1f5fdd2e096e..3b39fbfc8fe4 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1077,11 +1077,11 @@ static int unshare_sighand(struct task_struct *me)
*/
void __set_task_comm(struct task_struct *tsk, const char *buf, bool exec)
{
- size_t len = min(strlen(buf), sizeof(tsk->comm) - 1);
+ size_t len = min(strlen(buf), sizeof(tsk->comm_ext) - 1);

trace_task_rename(tsk, buf);
- memcpy(tsk->comm, buf, len);
- memset(&tsk->comm[len], 0, sizeof(tsk->comm) - len);
+ memcpy(tsk->comm_ext, buf, len);
+ memset(&tsk->comm_ext[len], 0, sizeof(tsk->comm_ext) - len);
perf_event_comm(tsk, exec);
}

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 704222114dcc..2605207170b4 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -316,6 +316,7 @@ struct user_event_mm;
*/
enum {
TASK_COMM_LEN = 16,
+ TASK_COMM_EXT_LEN = 64,
};

extern void sched_tick(void);
@@ -1165,7 +1166,10 @@ struct task_struct {
* - logic inside set_task_comm() will ensure it is always NUL-terminated and
* zero-padded
*/
- char comm[TASK_COMM_LEN];
+ union {
+ char comm[TASK_COMM_LEN];
+ char comm_ext[TASK_COMM_EXT_LEN];
+ };

struct nameidata *nameidata;

@@ -2005,7 +2009,7 @@ extern void __set_task_comm(struct task_struct *tsk, const char *from, bool exec
*/
#define get_task_comm(buf, tsk) ({ \
BUILD_BUG_ON(sizeof(buf) < TASK_COMM_LEN); \
- strscpy_pad(buf, (tsk)->comm); \
+ strscpy_pad(buf, (tsk)->comm_ext); \
buf; \
})

--
2.38.1