Re: [PATCH] printk: Fix incorrect length from print_time() when seconds > 99999

[Joe Perches <joe@perches.com>]

On Sat, 2012-12-29 at 09:56 -0800, Greg Kroah-Hartman wrote:
> On Fri, Dec 28, 2012 at 08:23:04PM -0800, Roland Dreier wrote:
> > From: Roland Dreier <roland@purestorage.com>
> > 
> > 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)
> > 
> > Signed-off-by: Roland Dreier <roland@purestorage.com>
> 
> Nice work.  When did you start seeing this problem, 3.6 or so?  I ask as
> it's probably something that should go to stable as well if so.
> 
> Andrew seems to be keeping the printk patches these days, so I'll let
> him pick this up with:

Sylvan Munaut did something similar
https://lkml.org/lkml/2012/12/5/168

It's been around quite awhile.