Re: [PATCH v4] string.h: Add str_has_prefix() helper function

[Steven Rostedt <rostedt@goodmis.org>]

On Sun, 23 Dec 2018 01:46:05 +0900
Namhyung Kim <namhyung@kernel.org> wrote:


> > What I meant by that is if a string is allocated at a end of a page,
> > and the next page is marked as not present. A read into that page will
> > cause a page fault, and since memcmp() does not stop at the '\0' it
> > will read into that not-present memory and trigger a fault, and that
> > read wont be in the exception table, and it will then BUG.  
> 
> Why it doesn't stop at the '\0' if one has it and the other doesn't?
> It's not because it's '\0', it's because they are different.  The '\0'
> should be in the prev page (otherwise it's already a BUG) so it should
> be detected and stopped before going to next page IMHO.
> 

Because memcmp() isn't required to test byte by byte. In fact, most
implementations don't which is why memcmp is faster than strcncmp.

It can be checking in 8 byte chunks or more (although perhaps not
likely). Perhaps there's an arch command that lets you compare 32 bytes
at a time, if the size passed to memcmp is 32 or more, the
implementation is allowed to read both src and dst of 32 bytes at a
time. If there was a '\0' followed by not present memory, you will
still get that fault.

-- Steve