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Date: Thu, 07 Jan 2021 16:51:31 -0800 From: Joe Perches <joe@...ches.com> To: Kees Cook <keescook@...omium.org> Cc: Dwaipayan Ray <dwaipayanray1@...il.com>, Jonathan Corbet <corbet@....net>, linux-kernel-mentees@...ts.linuxfoundation.org, linux-kernel <linux-kernel@...r.kernel.org>, Lukas Bulwahn <lukas.bulwahn@...il.com>, Linus Torvalds <torvalds@...ux-foundation.org> Subject: Re: deprecated.rst: deprecated strcpy ? (was: [PATCH] checkpatch: add a new check for strcpy/strlcpy uses) On Thu, 2021-01-07 at 13:16 -0800, Kees Cook wrote: > On Tue, Jan 05, 2021 at 01:28:18AM -0800, Joe Perches wrote: > > On Tue, 2021-01-05 at 14:29 +0530, Dwaipayan Ray wrote: > > > On Tue, Jan 5, 2021 at 2:14 PM Joe Perches <joe@...ches.com> wrote: > > > > > > > > On Tue, 2021-01-05 at 13:53 +0530, Dwaipayan Ray wrote: > > > > > strcpy() performs no bounds checking on the destination buffer. > > > > > This could result in linear overflows beyond the end of the buffer. > > > > > > > > > > strlcpy() reads the entire source buffer first. This read > > > > > may exceed the destination size limit. This can be both inefficient > > > > > and lead to linear read overflows. > > > > > > > > > > The safe replacement to both of these is to use strscpy() instead. > > > > > Add a new checkpatch warning which alerts the user on finding usage of > > > > > strcpy() or strlcpy(). > > > > > > > > I do not believe that strscpy is preferred over strcpy. > > > > > > > > When the size of the output buffer is known to be larger > > > > than the input, strcpy is faster. > > > > > > > > There are about 2k uses of strcpy. > > > > Is there a use where strcpy use actually matters? > > > > I don't know offhand... > > > > > > > > But I believe compilers do not optimize away the uses of strscpy > > > > to a simple memcpy like they do for strcpy with a const from > > > > > > > > strcpy(foo, "bar"); > > > > > > > > > > Yes the optimization here definitely helps. So in case the programmer > > > knows that the destination buffer is always larger, then strcpy() should be > > > preferred? I think the documentation might have been too strict about > > > strcpy() uses here: > > > > > > Documentation/process/deprecated.rst: > > > "strcpy() performs no bounds checking on the destination buffer. This > > > could result in linear overflows beyond the end of the buffer, leading to > > > all kinds of misbehaviors. While `CONFIG_FORTIFY_SOURCE=y` and various > > > compiler flags help reduce the risk of using this function, there is > > > no good reason to add new uses of this function. The safe replacement > > > is strscpy(),..." > > > > Kees/Jonathan: > > > > Perhaps this text is overly restrictive. > > > > There are ~2k uses of strcpy in the kernel. > > > > About half of these are where the buffer length of foo is known and the > > use is 'strcpy(foo, "bar")' so the compiler converts/optimizes away the > > strcpy to memcpy and may not even put "bar" into the string table. > > > > I believe strscpy uses do not have this optimization. > > > > Is there a case where the runtime costs actually matters? > > I expect so. > > The original goal was to use another helper that worked on static > strings like this. Linus rejected that idea, so we're in a weird place. > I think we could perhaps build a strcpy() replacement that requires > compile-time validated arguments, and to break the build if not. > > i.e. > > given: > char array[8]; > char *ptr; > > allow: > > > strcpy(array, "1234567"); > > disallow: > > strcpy(array, "12345678"); /* too long */ > strcpy(array, src); /* not optimized, so use strscpy? */ > strcpy(ptr, "1234567"); /* unknown destination size */ > strcpy(ptr, src); /* unknown destination size */ I think that's not a good idea as it's not a generic equivalent of the string.h code. I still like the stracpy variant I proposed: https://lore.kernel.org/lkml/24bb53c57767c1c2a8f266c305a670f7@sk2.org/T/#m0627aa770a076af1937cb5c610ed71dab3f1da72 https://lore.kernel.org/lkml/CAHk-=wgqQKoAnhmhGE-2PBFt7oQs9LLAATKbYa573UO=DPBE0Q@mail.gmail.com/ Linus liked a variant he called copy_string: https://lore.kernel.org/lkml/CAHk-=wg8vLmmwTGhXM51NpSWJW8RFEAKoXxG0Hu_Q9Uwbjj8kw@mail.gmail.com/ I think the cocci scripts that convert: strlcpy -> strscpy (only when return value unused) str<sln>cpy(array, "string") -> stracpy(foo, "string") s[cn]printf -> sysfs_emit would leave relatively few uses of strcpy and sprintf variants and would make it much easier to analyze the remainder uses for potential overflows.
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