Hi Kees,
On 4/3/25 10:00 PM, Kees Cook wrote:
On Mon, Mar 31, 2025 at 05:48:18PM +0530, Bhupesh wrote:
Provide a parallel implementation for get_task_comm() called
get_task_full_name() which allows the dynamically allocated
and filled-in task's full name to be passed to interested
users such as 'gdb'.
Currently while running 'gdb', the 'task->comm' value of a long
task name is truncated due to the limitation of TASK_COMM_LEN.
For example using gdb to debug a simple app currently which generate
threads with long task names:
# gdb ./threadnames -ex "run info thread" -ex "detach" -ex "quit" > log
# cat log
NameThatIsTooLo
This patch does not touch 'TASK_COMM_LEN' at all, i.e.
'TASK_COMM_LEN' and the 16-byte design remains untouched. Which means
that all the legacy / existing ABI, continue to work as before using
'/proc/$pid/task/$tid/comm'.
This patch only adds a parallel, dynamically-allocated
'task->full_name' which can be used by interested users
via '/proc/$pid/task/$tid/full_name'.
After this change, gdb is able to show full name of the task:
# gdb ./threadnames -ex "run info thread" -ex "detach" -ex "quit" > log
# cat log
NameThatIsTooLongForComm[4662]
Signed-off-by: Bhupesh <bhupesh@xxxxxxxxxx>
---
fs/exec.c | 21 ++++++++++++++++++---
include/linux/sched.h | 9 +++++++++
2 files changed, 27 insertions(+), 3 deletions(-)
diff --git a/fs/exec.c b/fs/exec.c
index f45859ad13ac..4219d77a519c 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1208,6 +1208,9 @@ int begin_new_exec(struct linux_binprm * bprm)
{
struct task_struct *me = current;
int retval;
+ va_list args;
+ char *name;
+ const char *fmt;
/* Once we are committed compute the creds */
retval = bprm_creds_from_file(bprm);
@@ -1348,11 +1351,22 @@ int begin_new_exec(struct linux_binprm * bprm)
* detecting a concurrent rename and just want a terminated name.
*/
rcu_read_lock();
- __set_task_comm(me, smp_load_acquire(&bprm->file->f_path.dentry->d_name.name),
- true);
+ fmt = smp_load_acquire(&bprm->file->f_path.dentry->d_name.name);
+ name = kvasprintf(GFP_KERNEL, fmt, args);
+ if (!name)
+ return -ENOMEM;
+
+ me->full_name = name;
+ __set_task_comm(me, fmt, true);
I don't want to add new allocations to the default exec path unless we
absolutely must.
In the original proposal this was about setting thread names (after
exec), and I think that'll be fine.
Ok.
rcu_read_unlock();
} else {
- __set_task_comm(me, kbasename(bprm->filename), true);
+ fmt = kbasename(bprm->filename);
+ name = kvasprintf(GFP_KERNEL, fmt, args);
+ if (!name)
+ return -ENOMEM;
+
+ me->full_name = name;
+ __set_task_comm(me, fmt, true);
}
I think we can just set me->full_name = me->comm by default.
Sure.
/* An exec changes our domain. We are no longer part of the thread
@@ -1399,6 +1413,7 @@ int begin_new_exec(struct linux_binprm * bprm)
return 0;
out_unlock:
+ kfree(me->full_name);
up_write(&me->signal->exec_update_lock);
if (!bprm->cred)
mutex_unlock(&me->signal->cred_guard_mutex);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 56ddeb37b5cd..053b52606652 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1166,6 +1166,9 @@ struct task_struct {
*/
char comm[TASK_COMM_LEN];
+ /* To store the full name if task comm is truncated. */
+ char *full_name;
+
struct nameidata *nameidata;
#ifdef CONFIG_SYSVIPC
@@ -2007,6 +2010,12 @@ extern void __set_task_comm(struct task_struct *tsk, const char *from, bool exec
buf; \
})
+#define get_task_full_name(buf, buf_size, tsk) ({ \
+ BUILD_BUG_ON(sizeof(buf) < TASK_COMM_LEN); \
+ strscpy_pad(buf, (tsk)->full_name, buf_size); \
+ buf; \
+})
I think it should be possible to just switch get_task_comm() to use
(tsk)->full_name.
In another review for this series, Yafang mentioned the following
cleanup + approach suggested by Linus (see [0]).
Also I have summarized my understanding on the basis of the suggestions
Linus shared and the accompanying background threads (please see [1]).
Kindly share your views on the same, so that I can change the
implementation in v3 series accordingly.
[0].https://lore.kernel.org/all/CAHk-=wjAmmHUg6vho1KjzQi2=psR30+CogFd4aXrThr2gsiS4g@xxxxxxxxxxxxxx/
[1]. https://lore.kernel.org/all/20250331121820.455916-1-bhupesh@xxxxxxxxxx/T/#m7c163829440f2c98e64b475b7cdbc35ba4d613ca
Thanks,
Bhupesh
