Re: [PATCH v2] lib/string.c: implement stpcpy

[Joe Perches <joe@perches.com>]

On Sat, 2020-08-15 at 13:47 -0700, Nick Desaulniers wrote:
> On Sat, Aug 15, 2020 at 9:34 AM Kees Cook <keescook@chromium.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.

Something like:

---
 include/linux/compiler-clang.h | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
index cee0c728d39a..6279f1904e39 100644
--- a/include/linux/compiler-clang.h
+++ b/include/linux/compiler-clang.h
@@ -61,3 +61,30 @@
 #if __has_feature(shadow_call_stack)
 # define __noscs	__attribute__((__no_sanitize__("shadow-call-stack")))
 #endif
+
+#ifndef __HAVE_ARCH_STPCPY
+/**
+ * stpcpy - copy a string from src to dest returning a pointer to the new end
+ *          of dest, including src's NULL terminator. May overrun dest.
+ * @dest: pointer to buffer being copied into.
+ *        Must be large enough to receive copy.
+ * @src: pointer to the beginning of string being copied from.
+ *       Must not overlap dest.
+ *
+ * This function exists _only_ to support clang's possible conversion of
+ * sprintf calls to stpcpy.
+ *
+ * stpcpy differs from strcpy in two key ways:
+ * 1. inputs must not overlap.
+ * 2. return value is dest's NUL termination character after copy.
+ *    (for strcpy, the return value is a pointer to src)
+ */
+
+static inline char *stpcpy(char __restrict *dest, const char __restrict *src)
+{
+	while ((*dest++ = *src++) != '\0') {
+		;	/* nothing */
+	}
+       return --dest;
+}
+#endif