Messages in this thread | | | Date | Thu, 7 Jan 2021 13:16:26 -0800 | From | Kees Cook <> | Subject | Re: deprecated.rst: deprecated strcpy ? (was: [PATCH] checkpatch: add a new check for strcpy/strlcpy uses) |
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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@perches.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 */
What do you think?
-- Kees Cook
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