path: root/kernel
diff options
authorRoland Dreier <roland@purestorage.com>2013-01-04 15:35:50 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2013-01-04 16:11:48 -0800
commit35dac27cedd14c3b6fcd4ba7bc3c31738cfd1831 (patch)
tree22defb106f9c4081a14bb0fd54a6740b0460fc36 /kernel
parent2f90b68309683f2c5765a1b04ca23d71e51f1494 (diff)
printk: fix incorrect length from print_time() when seconds > 99999
print_prefix() passes a NULL buf to print_time() to get the length of the time prefix; when printk times are enabled, the current code just returns the constant 15, which matches the format "[%5lu.%06lu] " used to print the time value. However, this is obviously incorrect when the whole seconds part of the time gets beyond 5 digits (100000 seconds is a bit more than a day of uptime). The simple fix is to use snprintf(NULL, 0, ...) to calculate the actual length of the time prefix. This could be micro-optimized but it seems better to have simpler, more readable code here. The bug leads to the syslog system call miscomputing which messages fit into the userspace buffer. If there are enough messages to fill log_buf_len and some have a timestamp >= 100000, dmesg may fail with: # dmesg klogctl: Bad address When this happens, strace shows that the failure is indeed EFAULT due to the kernel mistakenly accessing past the end of dmesg's buffer, since dmesg asks the kernel how big a buffer it needs, allocates a bit more, and then gets an error when it asks the kernel to fill it: syslog(0xa, 0, 0) = 1048576 mmap(NULL, 1052672, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fa4d25d2000 syslog(0x3, 0x7fa4d25d2010, 0x100008) = -1 EFAULT (Bad address) As far as I can see, the bug has been there as long as print_time(), which comes from commit 084681d14e42 ("printk: flush continuation lines immediately to console") in 3.5-rc5. Signed-off-by: Roland Dreier <roland@purestorage.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Joe Perches <joe@perches.com> Cc: Sylvain Munaut <s.munaut@whatever-company.com> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'kernel')
1 files changed, 3 insertions, 2 deletions
diff --git a/kernel/printk.c b/kernel/printk.c
index 19c0d7bcf24..357f714ddd4 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -870,10 +870,11 @@ static size_t print_time(u64 ts, char *buf)
if (!printk_time)
return 0;
+ rem_nsec = do_div(ts, 1000000000);
if (!buf)
- return 15;
+ return snprintf(NULL, 0, "[%5lu.000000] ", (unsigned long)ts);
- rem_nsec = do_div(ts, 1000000000);
return sprintf(buf, "[%5lu.%06lu] ",
(unsigned long)ts, rem_nsec / 1000);