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Subject[PATCH 2/2] procfs: let userspace find out max /proc/pid/comm length
Abuse llseek(SEEK_END) to return max /proc/pid/comm length.

This is needed for pthread_getname_np to be able to return ERANGE
without modifying thread's name.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
---
fs/proc/base.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 7238a64751e4..48089f087a8e 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1542,11 +1542,20 @@ static int comm_open(struct inode *inode, struct file *filp)
return single_open(filp, comm_show, inode);
}

+static loff_t comm_lseek(struct file *file, loff_t offset, int whence)
+{
+ /* SEEK_END for seq_files normally gets -EINVAL */
+ if (whence == SEEK_END && offset == 0)
+ return TASK_COMM_LEN - 1;
+
+ return seq_lseek(file, offset, whence);
+}
+
static const struct file_operations proc_pid_set_comm_operations = {
.open = comm_open,
.read = seq_read,
.write = comm_write,
- .llseek = seq_lseek,
+ .llseek = comm_lseek,
.release = single_release,
};

--
2.11.0
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