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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id v3sm6349833pjn.7.2021.01.07.13.16.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 07 Jan 2021 13:16:27 -0800 (PST) Date: Thu, 7 Jan 2021 13:16:26 -0800 From: Kees Cook To: Joe Perches Message-ID: <202101071310.3AC5F0C4@keescook> References: <20210105082303.15310-1-dwaipayanray1@gmail.com> <50cc861121b62b3c1518222f24f679c3f72b868d.camel@perches.com> <3ffe616d8c3fb54833bfc4d86cb73427cf6c7add.camel@perches.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <3ffe616d8c3fb54833bfc4d86cb73427cf6c7add.camel@perches.com> Cc: Dwaipayan Ray , linux-kernel-mentees@lists.linuxfoundation.org, linux-kernel , Jonathan Corbet Subject: Re: [Linux-kernel-mentees] deprecated.rst: deprecated strcpy ? (was: [PATCH] checkpatch: add a new check for strcpy/strlcpy uses) X-BeenThere: linux-kernel-mentees@lists.linuxfoundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Errors-To: linux-kernel-mentees-bounces@lists.linuxfoundation.org Sender: "Linux-kernel-mentees" 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 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 > > > = > > > =A0=A0=A0=A0=A0=A0=A0=A0strcpy(foo, "bar"); > > > = > > = > > Yes the optimization here definitely helps. So in case the programmer > > knows that the destination buffer is always larger, then strcpy() shoul= d 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=3Dy` 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 _______________________________________________ Linux-kernel-mentees mailing list Linux-kernel-mentees@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/linux-kernel-mentees