The patch titled Subject: printk: fix incorrect length from print_time() when seconds > 99999 has been added to the -mm tree. Its filename is printk-fix-incorrect-length-from-print_time-when-seconds-99999.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/SubmitChecklist when testing your code *** The -mm tree is included into linux-next and is updated there every 3-4 working days ------------------------------------------------------ From: Roland Dreier <roland@xxxxxxxxxxxxxxx> Subject: 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 084681d14e42 ("printk: flush continuation lines immediately to console") in 3.5-rc5. Signed-off-by: Roland Dreier <roland@xxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> Cc: Joe Perches <joe@xxxxxxxxxxx> Cc: Sylvain Munaut <s.munaut@xxxxxxxxxxxxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- kernel/printk.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff -puN kernel/printk.c~printk-fix-incorrect-length-from-print_time-when-seconds-99999 kernel/printk.c --- a/kernel/printk.c~printk-fix-incorrect-length-from-print_time-when-seconds-99999 +++ a/kernel/printk.c @@ -871,10 +871,11 @@ static size_t print_time(u64 ts, char *b 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); } _ Patches currently in -mm which might be from roland@xxxxxxxxxxxxxxx are printk-fix-incorrect-length-from-print_time-when-seconds-99999.patch -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html