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Date: Sat, 15 Aug 2020 14:31:35 -0700 From: Joe Perches <joe@...ches.com> To: Nick Desaulniers <ndesaulniers@...gle.com> Cc: Kees Cook <keescook@...omium.org>, Andrew Morton <akpm@...ux-foundation.org>, Dávid Bolvanský <david.bolvansky@...il.com>, Eli Friedman <efriedma@...cinc.com>, "# 3.4.x" <stable@...r.kernel.org>, Arvind Sankar <nivedita@...m.mit.edu>, Sami Tolvanen <samitolvanen@...gle.com>, Vishal Verma <vishal.l.verma@...el.com>, Dan Williams <dan.j.williams@...el.com>, Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, "Joel Fernandes (Google)" <joel@...lfernandes.org>, Daniel Axtens <dja@...ens.net>, Ingo Molnar <mingo@...nel.org>, Yury Norov <yury.norov@...il.com>, Alexandru Ardelean <alexandru.ardelean@...log.com>, LKML <linux-kernel@...r.kernel.org>, clang-built-linux <clang-built-linux@...glegroups.com>, Rasmus Villemoes <linux@...musvillemoes.dk> Subject: Re: [PATCH v2] lib/string.c: implement stpcpy On Sat, 2020-08-15 at 14:28 -0700, Nick Desaulniers wrote: > On Sat, Aug 15, 2020 at 2:24 PM Joe Perches <joe@...ches.com> wrote: > > On Sat, 2020-08-15 at 13:47 -0700, Nick Desaulniers wrote: > > > On Sat, Aug 15, 2020 at 9:34 AM Kees Cook <keescook@...omium.org> wrote: > > > > On Fri, Aug 14, 2020 at 07:09:44PM -0700, Nick Desaulniers wrote: > > > > > LLVM implemented a recent "libcall optimization" that lowers calls to > > > > > `sprintf(dest, "%s", str)` where the return value is used to > > > > > `stpcpy(dest, str) - dest`. This generally avoids the machinery involved > > > > > in parsing format strings. Calling `sprintf` with overlapping arguments > > > > > was clarified in ISO C99 and POSIX.1-2001 to be undefined behavior. > > > > > > > > > > `stpcpy` is just like `strcpy` except it returns the pointer to the new > > > > > tail of `dest`. This allows you to chain multiple calls to `stpcpy` in > > > > > one statement. > > > > > > > > O_O What? > > > > > > > > No; this is a _terrible_ API: there is no bounds checking, there are no > > > > buffer sizes. Anything using the example sprintf() pattern is _already_ > > > > wrong and must be removed from the kernel. (Yes, I realize that the > > > > kernel is *filled* with this bad assumption that "I'll never write more > > > > than PAGE_SIZE bytes to this buffer", but that's both theoretically > > > > wrong ("640k is enough for anybody") and has been known to be wrong in > > > > practice too (e.g. when suddenly your writing routine is reachable by > > > > splice(2) and you may not have a PAGE_SIZE buffer). > > > > > > > > But we cannot _add_ another dangerous string API. We're already in a > > > > terrible mess trying to remove strcpy[1], strlcpy[2], and strncpy[3]. This > > > > needs to be addressed up by removing the unbounded sprintf() uses. (And > > > > to do so without introducing bugs related to using snprintf() when > > > > scnprintf() is expected[4].) > > > > > > Well, everything (-next, mainline, stable) is broken right now (with > > > ToT Clang) without providing this symbol. I'm not going to go clean > > > the entire kernel's use of sprintf to get our CI back to being green. > > > > Maybe this should get place in compiler-clang.h so it isn't > > generic and public. > > https://bugs.llvm.org/show_bug.cgi?id=47162#c7 and > https://bugs.llvm.org/show_bug.cgi?id=47144 > Seem to imply that Clang is not the only compiler that can lower a > sequence of libcalls to stpcpy. Do we want to wait until we have a > fire drill w/ GCC to move such an implementation from > include/linux/compiler-clang.h back in to lib/string.c? My guess is yes, wait until gcc, if ever, needs it.
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