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Message-ID: <202209011123.27D7D67@keescook> Date: Thu, 1 Sep 2022 11:35:17 -0700 From: Kees Cook <keescook@...omium.org> To: Geert Uytterhoeven <geert@...ux-m68k.org> Cc: Wolfram Sang <wsa+renesas@...g-engineering.com>, Nick Desaulniers <ndesaulniers@...gle.com>, Guenter Roeck <linux@...ck-us.net>, Paolo Abeni <pabeni@...hat.com>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, linux-hardening@...r.kernel.org, Linus Torvalds <torvalds@...ux-foundation.org> Subject: Re: [PATCH] string: Introduce strtomem() and strtomem_pad() On Thu, Sep 01, 2022 at 10:39:19AM +0200, Geert Uytterhoeven wrote: > [...] > > When the "__nonstring" attributes are missing, the intent of the > > programmer becomes ambiguous for whether the lack of a trailing NUL > > in the p.small copy is a bug. Additionally, it's not clear whether > > the trailing padding in the p.big copy is _needed_. Both cases > > become unambiguous with: > > > > strtomem(p.small, "hello"); > > strtomem_pad(p.big, "hello"); > > strtomem_pad(p.big, "hello", 0); Oops, thanks. I will adjust the example. And actually, instead of these notes just living in commit logs, I realize I can update the kerndoc for strncpy with a "here's now to pick a replacement" table... > > See also https://github.com/KSPP/linux/issues/90 > > > > Expand the memcpy KUnit tests to include these functions. > > > > Cc: Wolfram Sang <wsa+renesas@...g-engineering.com> > > Cc: Nick Desaulniers <ndesaulniers@...gle.com> > > Cc: Geert Uytterhoeven <geert@...ux-m68k.org> > > Cc: Guenter Roeck <linux@...ck-us.net> > > Signed-off-by: Kees Cook <keescook@...omium.org> > > The idea looks good to me, but I guess Linus has something to > say, too. > > > --- a/include/linux/string.h > > +++ b/include/linux/string.h > > @@ -260,6 +260,49 @@ static inline const char *kbasename(const char *path) > > void memcpy_and_pad(void *dest, size_t dest_len, const void *src, size_t count, > > int pad); > > > > +/** > > + * strtomem_pad - Copy NUL-terminated string to non-NUL-terminated buffer > > + * > > + * @dest: Pointer of destination character array (marked as __nonstring) > > + * @src: Pointer to NUL-terminated string > > + * @pad: Padding character to fill any remaining bytes of @dest after copy > > + * > > + * This is a replacement for strncpy() uses where the destination is not > > + * a NUL-terminated string, but with bounds checking on the source size, and > > + * an explicit padding character. If padding is not required, use strtomem(). > > + * > > + * Note that the size of @dest is not an argument, as the length of @dest > > + * must be discoverable by the compiler. > > + */ > > +#define strtomem_pad(dest, src, pad) do { \ > > + const size_t _dest_len = __builtin_object_size(dest, 1); \ > > + \ > > + BUILD_BUG_ON(!__builtin_constant_p(_dest_len) || \ > > + _dest_len == (size_t)-1); \ > > I think you want to include __must_be_array(dest) here. I didn't do that for the cases where we may be writing to non-array destinations (e.g. see the cast from u64 in the strncpy use in tools/perf/arch/x86/util/intel-pt.c). Since what we need to know is the object size, it does not strictly need to be an array. > > [...] > > + memset(&wrap, 0xFF, sizeof(wrap)); > > + KUNIT_EXPECT_EQ_MSG(test, wrap.canary1, -1UL, > > -1L or ULONG_MAX (everywhere) Yeah, ULONG_MAX looks best. Thanks! > > > + "bad initial canary value"); > > + KUNIT_EXPECT_EQ_MSG(test, wrap.canary2, -1UL, > > + "bad initial canary value"); > > + > > + /* Check unpadded copy leaves surroundings untouched. */ > > + strtomem(wrap.output, input); > > + KUNIT_EXPECT_EQ(test, wrap.canary1, -1UL); > > + KUNIT_EXPECT_EQ(test, wrap.output[0], input[0]); > > + KUNIT_EXPECT_EQ(test, wrap.output[1], input[1]); > > + for (int i = 2; i < sizeof(wrap.output); i++) > > unsigned int i (everywhere) I guess, but why? This could even be u8. Thanks for the review! -- Kees Cook
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