Messages in this thread | | | Date | Mon, 5 Oct 2015 15:15:24 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] string: Improve the generic strlcpy() implementation |
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* Ingo Molnar <mingo@kernel.org> wrote:
> > * Ingo Molnar <mingo@kernel.org> wrote: > > > 2) > > > > Another problem is that strlcpy() will also happily do bad stuff if we pass > > it a negative size. Instead of that we will from now on print a (one time) > > warning and return safely. > > Hm, so this check is buggy, as 'size_t' is unsigned - and for some reason GCC > didn't warn about the never-met comparison and the resulting unreachable dead > code here: > > > + /* Overflow check: */ > > + if (unlikely(dest_size < 0)) { > > + WARN_ONCE(1, "strlcpy(): dest_size < 0 underflow!"); > > + return strlen(src); > > + } > > which is annoying. > > Would people object to something like: > > > + /* Overflow check: */ > > + if (unlikely((ssize_t)dest_size < 0)) { > > + WARN_ONCE(1, "strlcpy(): dest_size < 0 underflow!"); > > + return strlen(src); > > + } > > ? > > As I doubt it's legit to have larger than 2GB strings. > > Also, I'm wondering why GCC didn't warn.
Hm, so GCC (v4.9.2) will only warn about this bug if -Wtype-limits is enabled explicitly:
lib/string.c: In function ‘strlcpy’: lib/string.c:228:32: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits] if (unlikely((size_t)dst_size < 0)) { ^
... which we don't do in the kernel.
Has anyone considered enabling -Wtype-limits? It seems to catch real bugs.
I can see there are patches that enable -Wextra (which enables -Wtype-limits and many other warnings), but it would be more manageable to just enable one such warning at a time.
Thanks,
Ingo
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