* Linus Torvalds <torva...@linux-foundation.org> wrote:
> So I really refuse to worry about the snprintf() family of functions wrt this
> race. I don't think it was hugely important for strlcpy() either - more of a
> "quality of implementation" issue rather than anything fundamental - but for
> snprintf and friends it's an almost unavoidable issue because of how snprintf
> works.
>
> Saying that 'strlcpy()' and 'snprintf("%s")' are equivalent is true only in
> the
> loosest sense. Yes, they return the same return value. Yes, the result string
> should be the same. But the two are completely different despite that.
>
> snprintf() has to handle all the *other* cases than just "%s", including
> right-justification, string precision handling, etc etc. It is effectively
> impossible to do without doing "strlen()" on the source of the string
> beforehand. As a result, snprintf() is fundamentally always going to be racy
> wrt
> the string changing during the call.
>
> So the simple end result is that we shouldn't worry about it, and if you are
> doing snprintf() on a changing string, you should just be aware of it. We
> *do*
> actually do that, for things like "current->comm" that really can change
> while
> being printed out. We just don't care deeply, and have in fact been removing
> locks in this area, because the end result is still guaranteed to be
> NUL-terminated etc.
>
> Can we get odd truncated printouts in the (very very very unlikely) case that
> the string is being changed? Yes. We just don't care.
I do agree mostly, but I think we should still try to achieve the following two
properties, if possible sanely+cheaply+cleanly:
- the printed string should not contain spurious \0 bytes even if the %s source
'races'. [I think this is true currently.]
- the return code should correctly represent what snprintf did to the target
string. [This might not be the case currently. But I'm not sure!]
Because that's a real concern I think: snprintf() return is used frequently to
iterate over buffers, and it should correctly and reliably represent what it
did,
regardless of what the source buffer does - because snprintf obviously knows
what
it did to the output buffer, it has full, race-free control over it.
Whether left-alignment and other formatting details were calculated correctly,
etc. is a secondary concern and cannot be guaranteed, but we should at least
guarantee that we generated a single string, that we did nothing else, and that
we
correctly returned its length.
Agreed?
Thanks,
Ingo
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