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authorBruno Meneguele <bmeneg@redhat.com>2020-03-17 07:33:44 -0300
committerPetr Mladek <pmladek@suse.com>2020-05-21 13:32:25 +0200
commit8ece3b3eb576a78d2e67ad4c3a80a39fa6708809 (patch)
treeadbc01ab55fd5dd70eb61e77391fa002670490c6 /kernel/printk
parent325606af573152e02f44d791f152b7f9564bcb30 (diff)
kernel/printk: add kmsg SEEK_CUR handling
Userspace libraries, e.g. glibc's dprintf(), perform a SEEK_CUR operation over any file descriptor requested to make sure the current position isn't pointing to junk due to previous manipulation of that same fd. And whenever that fd doesn't have support for such operation, the userspace code expects -ESPIPE to be returned. However, when the fd in question references the /dev/kmsg interface, the current kernel code state returns -EINVAL instead, causing an unexpected behavior in userspace: in the case of glibc, when -ESPIPE is returned it gets ignored and the call completes successfully, while returning -EINVAL forces dprintf to fail without performing any action over that fd: if (_IO_SEEKOFF (fp, (off64_t)0, _IO_seek_cur, _IOS_INPUT|_IOS_OUTPUT) == _IO_pos_BAD && errno != ESPIPE) return NULL; With this patch we make sure to return the correct value when SEEK_CUR is requested over kmsg and also add some kernel doc information to formalize this behavior. Link: https://lore.kernel.org/r/20200317103344.574277-1-bmeneg@redhat.com Cc: linux-kernel@vger.kernel.org Cc: rostedt@goodmis.org, Cc: David.Laight@ACULAB.COM Signed-off-by: Bruno Meneguele <bmeneg@redhat.com> Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> Signed-off-by: Petr Mladek <pmladek@suse.com>
Diffstat (limited to 'kernel/printk')
-rw-r--r--kernel/printk/printk.c10
1 files changed, 10 insertions, 0 deletions
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 525038745a14..35cc5f548860 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -974,6 +974,16 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
user->idx = log_next_idx;
user->seq = log_next_seq;
break;
+ case SEEK_CUR:
+ /*
+ * It isn't supported due to the record nature of this
+ * interface: _SET _DATA and _END point to very specific
+ * record positions, while _CUR would be more useful in case
+ * of a byte-based log. Because of that, return the default
+ * errno value for invalid seek operation.
+ */
+ ret = -ESPIPE;
+ break;
default:
ret = -EINVAL;
}