| From 5f4ef1af278d832d9d0d49d56a57b998919c56b9 Mon Sep 17 00:00:00 2001 |
| From: Manoj Gupta <manojgupta@google.com> |
| Date: Fri, 11 Sep 2020 17:10:38 -0700 |
| Subject: [PATCH] FROMLIST: lib/string.c: implement stpcpy |
| |
| 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 H. 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. |
| |
| Link: https://lkml.kernel.org/r/20200825140001.2941001-1-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: Nick Desaulniers <ndesaulniers@google.com> |
| Reported-by: Sami Tolvanen <samitolvanen@google.com> |
| Tested-by: Nathan Chancellor <natechancellor@gmail.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: Masahiro Yamada <masahiroy@kernel.org> |
| Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk> |
| Acked-by: Kees Cook <keescook@chromium.org> |
| Tested-by: Sedat Dilek <sedat.dilek@gmail.com> |
| Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com> |
| Cc: Alexandru Ardelean <alexandru.ardelean@analog.com> |
| Cc: Yury Norov <yury.norov@gmail.com> |
| Cc: <stable@vger.kernel.org> |
| Signed-off-by: Andrew Morton <akpm@linux-foundation.org> |
| (am from https://lore.kernel.org/patchwork/patch/1295112/) |
| |
| BUG=chromium:1125877 |
| TEST=CQ |
| |
| Change-Id: Ieeea470bb7dc2ab46a66d20a4d37fe74e2d5c065 |
| Signed-off-by: Manoj Gupta <manojgupta@google.com> |
| Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/2407003 |
| Tested-by: Manoj Gupta <manojgupta@chromium.org> |
| Reviewed-by: Sean Paul <seanpaul@chromium.org> |
| Reviewed-by: Caroline Tice <cmtice@chromium.org> |
| Commit-Queue: Manoj Gupta <manojgupta@chromium.org> |
| --- |
| lib/string.c | 24 ++++++++++++++++++++++++ |
| 1 file changed, 24 insertions(+) |
| |
| diff --git a/lib/string.c b/lib/string.c |
| index 72125fd5b4a6..2622a7b7f6ef 100644 |
| --- a/lib/string.c |
| +++ b/lib/string.c |
| @@ -236,6 +236,30 @@ ssize_t strscpy(char *dest, const char *src, size_t count) |
| 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 the new |
| + * %NUL-terminated character. (for strcpy, the return value is a pointer to |
| + * src. 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 |
| -- |
| 2.29.2.454.gaff20da3a2-goog |
| |