--- /dev/null
+From 1e1b6d63d6340764e00356873e5794225a2a03ea Mon Sep 17 00:00:00 2001
+From: Nick Desaulniers <ndesaulniers@google.com>
+Date: Fri, 25 Sep 2020 21:19:18 -0700
+Subject: lib/string.c: implement stpcpy
+
+From: Nick Desaulniers <ndesaulniers@google.com>
+
+commit 1e1b6d63d6340764e00356873e5794225a2a03ea upstream.
+
+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.
+`stpcpy` is just like `strcpy` except it returns the pointer to the new
+tail of `dest`. This optimization was introduced into clang-12.
+
+Implement this so that we don't observe linkage failures due to missing
+symbol definitions for `stpcpy`.
+
+Similar to last year's fire drill with: commit 5f074f3e192f
+("lib/string.c: implement a basic bcmp")
+
+The kernel is somewhere between a "freestanding" environment (no full
+libc) and "hosted" environment (many symbols from libc exist with the
+same type, function signature, and semantics).
+
+As Peter Anvin notes, there's not really a great way to inform the
+compiler that you're targeting a freestanding environment but would like
+to opt-in to some libcall optimizations (see pr/47280 below), rather
+than opt-out.
+
+Arvind notes, -fno-builtin-* behaves slightly differently between GCC
+and Clang, and Clang is missing many __builtin_* definitions, which I
+consider a bug in Clang and am working on fixing.
+
+Masahiro summarizes the subtle distinction between compilers justly:
+ To prevent transformation from foo() into bar(), there are two ways in
+ Clang to do that; -fno-builtin-foo, and -fno-builtin-bar. There is
+ only one in GCC; -fno-buitin-foo.
+
+(Any difference in that behavior in Clang is likely a bug from a missing
+__builtin_* definition.)
+
+Masahiro also notes:
+ We want to disable optimization from foo() to bar(),
+ but we may still benefit from the optimization from
+ foo() into something else. If GCC implements the same transform, we
+ would run into a problem because it is not -fno-builtin-bar, but
+ -fno-builtin-foo that disables that optimization.
+
+ In this regard, -fno-builtin-foo would be more future-proof than
+ -fno-built-bar, but -fno-builtin-foo is still potentially overkill. We
+ may want to prevent calls from foo() being optimized into calls to
+ bar(), but we still may want other optimization on calls to foo().
+
+It seems that compilers today don't quite provide the fine grain control
+over which libcall optimizations pseudo-freestanding environments would
+prefer.
+
+Finally, Kees notes that this interface is unsafe, so we should not
+encourage its use. As such, I've removed the declaration from any
+header, but it still needs to be exported to avoid linkage errors in
+modules.
+
+Reported-by: Sami Tolvanen <samitolvanen@google.com>
+Suggested-by: Andy Lavr <andy.lavr@gmail.com>
+Suggested-by: Arvind Sankar <nivedita@alum.mit.edu>
+Suggested-by: Joe Perches <joe@perches.com>
+Suggested-by: Kees Cook <keescook@chromium.org>
+Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
+Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
+Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
+Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
+Tested-by: Nathan Chancellor <natechancellor@gmail.com>
+Cc: <stable@vger.kernel.org>
+Link: https://lkml.kernel.org/r/20200914161643.938408-1-ndesaulniers@google.com
+Link: https://bugs.llvm.org/show_bug.cgi?id=47162
+Link: https://bugs.llvm.org/show_bug.cgi?id=47280
+Link: https://github.com/ClangBuiltLinux/linux/issues/1126
+Link: https://man7.org/linux/man-pages/man3/stpcpy.3.html
+Link: https://pubs.opengroup.org/onlinepubs/9699919799/functions/stpcpy.html
+Link: https://reviews.llvm.org/D85963
+Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
+Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
+
+---
+ lib/string.c | 24 ++++++++++++++++++++++++
+ 1 file changed, 24 insertions(+)
+
+--- a/lib/string.c
++++ b/lib/string.c
+@@ -235,6 +235,30 @@ ssize_t strscpy(char *dest, const char *
+ EXPORT_SYMBOL(strscpy);
+ #endif
+
++/**
++ * stpcpy - copy a string from src to dest returning a pointer to the new end
++ * of dest, including src's %NUL-terminator. May overrun dest.
++ * @dest: pointer to end of string being copied into. Must be large enough
++ * to receive copy.
++ * @src: pointer to the beginning of string being copied from. Must not overlap
++ * dest.
++ *
++ * stpcpy differs from strcpy in a key way: the return value is a pointer
++ * to the new %NUL-terminating character in @dest. (For strcpy, the return
++ * value is a pointer to the start of @dest). This interface is considered
++ * unsafe as it doesn't perform bounds checking of the inputs. As such it's
++ * not recommended for usage. Instead, its definition is provided in case
++ * the compiler lowers other libcalls to stpcpy.
++ */
++char *stpcpy(char *__restrict__ dest, const char *__restrict__ src);
++char *stpcpy(char *__restrict__ dest, const char *__restrict__ src)
++{
++ while ((*dest++ = *src++) != '\0')
++ /* nothing */;
++ return --dest;
++}
++EXPORT_SYMBOL(stpcpy);
++
+ #ifndef __HAVE_ARCH_STRCAT
+ /**
+ * strcat - Append one %NUL-terminated string to another
projects / thirdparty / kernel / stable-queue.git / commitdiff
