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SubjectRe: Regression - /proc/kmsg does not (always) block for 1-byte reads
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On Sat, 2012-07-07 at 23:19 +0200, Kay Sievers wrote:
> On Fri, Jul 6, 2012 at 10:30 PM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> > Kay, this needs to be fixed.
> >
> > Suggested fix: just use the 'seq_printf()' interfaces, which do the
> > proper buffering, and allow any size reads of various packetized data.
>
> I'll have a look.

Hmm, we need to block in the read() when we have no data, and we need to
support concurrent readers where only one of them sees the data, and we
need O_NONBLOCK support. Maybe I miss something but the seq_file stuff
seems to get complicated, as it takes a mutex internally which gets in
the way of the O_NONBLOCK stuff.

Here is what seems to work for me. If the buffer is to small to fit the
first record, we deliver a partial record, and start from that offset
again with the next read().

I'll need to do more testing tomorrow.

Thanks,
Kay

---
kernel/printk.c | 28 ++++++++++++++++++++--------
1 file changed, 20 insertions(+), 8 deletions(-)

--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -217,6 +217,7 @@ static DEFINE_RAW_SPINLOCK(logbuf_lock);
/* the next printk record to read by syslog(READ) or /proc/kmsg */
static u64 syslog_seq;
static u32 syslog_idx;
+static size_t syslog_partial;

/* index and sequence number of the first record stored in the buffer */
static u64 log_first_seq;
@@ -890,22 +891,33 @@ static int syslog_print(char __user *buf

while (size > 0) {
size_t n;
+ size_t skip;

raw_spin_lock_irq(&logbuf_lock);
if (syslog_seq < log_first_seq) {
/* messages are gone, move to first one */
syslog_seq = log_first_seq;
syslog_idx = log_first_idx;
+ syslog_partial = 0;
}
if (syslog_seq == log_next_seq) {
raw_spin_unlock_irq(&logbuf_lock);
break;
}
+
+ skip = syslog_partial;
msg = log_from_idx(syslog_idx);
n = msg_print_text(msg, true, text, LOG_LINE_MAX);
- if (n <= size) {
+ if (n - syslog_partial <= size) {
+ /* message fits into buffer, move forward */
syslog_idx = log_next(syslog_idx);
syslog_seq++;
+ n -= syslog_partial;
+ syslog_partial = 0;
+ } else if (!len){
+ /* partial read(), remember position */
+ n = size;
+ syslog_partial += n;
} else
n = 0;
raw_spin_unlock_irq(&logbuf_lock);
@@ -913,17 +925,15 @@ static int syslog_print(char __user *buf
if (!n)
break;

- len += n;
- size -= n;
- buf += n;
- n = copy_to_user(buf - n, text, n);
-
- if (n) {
- len -= n;
+ if (copy_to_user(buf, text + skip, n)) {
if (!len)
len = -EFAULT;
break;
}
+
+ len += n;
+ size -= n;
+ buf += n;
}

kfree(text);
@@ -1107,6 +1117,7 @@ int do_syslog(int type, char __user *buf
/* messages are gone, move to first one */
syslog_seq = log_first_seq;
syslog_idx = log_first_idx;
+ syslog_partial = 0;
}
if (from_file) {
/*
@@ -1129,6 +1140,7 @@ int do_syslog(int type, char __user *buf
idx = log_next(idx);
seq++;
}
+ error -= syslog_partial;
}
raw_spin_unlock_irq(&logbuf_lock);
break;



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