From 625d3449788f85569096780592549d0340e9c0c7 Mon Sep 17 00:00:00 2001 From: "Jason A. Donenfeld" Date: Sun, 21 Jun 2020 21:02:22 -0600 Subject: [PATCH] Revert "kernel/printk: add kmsg SEEK_CUR handling" This reverts commit 8ece3b3eb576a78d2e67ad4c3a80a39fa6708809. This commit broke userspace. Bash uses ESPIPE to determine whether or not the file should be read using "unbuffered I/O", which means reading 1 byte at a time instead of 128 bytes at a time. I used to use bash to read through kmsg in a really quite nasty way: while read -t 0.1 -r line 2>/dev/null || [[ $? -ne 142 ]]; do echo "SARU $line" done < /dev/kmsg This will show all lines that can fit into the 128 byte buffer, and skip lines that don't. That's pretty awful, but at least it worked. With this change, bash now tries to do 1-byte reads, which means it skips all the lines, which is worse than before. Now, I don't really care very much about this, and I'm already look for a workaround. But I did just spend an hour trying to figure out why my scripts were broken. Either way, it makes no difference to me personally whether this is reverted, but it might be something to consider. If you declare that "trying to read /dev/kmsg with bash is terminally stupid anyway," I might be inclined to agree with you. But do note that bash uses lseek(fd, 0, SEEK_CUR)==>ESPIPE to determine whether or not it's reading from a pipe. Cc: Bruno Meneguele Cc: Sergey Senozhatsky Cc: Steven Rostedt Cc: David Laight Cc: Sergey Senozhatsky Cc: Petr Mladek Signed-off-by: Jason A. Donenfeld Signed-off-by: Linus Torvalds --- Documentation/ABI/testing/dev-kmsg | 5 ----- kernel/printk/printk.c | 10 ---------- 2 files changed, 15 deletions(-) diff --git a/Documentation/ABI/testing/dev-kmsg b/Documentation/ABI/testing/dev-kmsg index 1e6c28b..f307506e 100644 --- a/Documentation/ABI/testing/dev-kmsg +++ b/Documentation/ABI/testing/dev-kmsg @@ -56,11 +56,6 @@ Description: The /dev/kmsg character device node provides userspace access seek after the last record available at the time the last SYSLOG_ACTION_CLEAR was issued. - Due to the record nature of this interface with a "read all" - behavior and the specific positions each seek operation sets, - SEEK_CUR is not supported, returning -ESPIPE (invalid seek) to - errno whenever requested. - The output format consists of a prefix carrying the syslog prefix including priority and facility, the 64 bit message sequence number and the monotonic timestamp in microseconds, diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 8c14835..b71eaf5 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -974,16 +974,6 @@ 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; } -- 2.7.4