Aster Boese
b33ba8203c
The previous patch was broken because the kernel was still configured to use GCC. Builds passed in pmaports CI because the runner is native aarch64, which hits the issue of Alpine's clang packaging depending on gcc to get access to libgcc-dev. This pulls in the whole GCC compiler, causing the kernel to default to it and ignore clang. When this patch got to BPO, it failed because BPO cross-compiles packages, leading to aarch64 GCC not being installed when building (only clang is used as the cross-compiler) and causing the build to fail there. Signed-off-by: Aster Boese <asterboese@mailbox.org> Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/8265>
124 lines
5.2 KiB
Diff
124 lines
5.2 KiB
Diff
From 5934637641c863cc2c1765a0d01c5b6f53ecc4fc Mon Sep 17 00:00:00 2001
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From: Nick Desaulniers <ndesaulniers@google.com>
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Date: Wed, 26 Aug 2020 15:21:29 +1000
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Subject: lib/string.c: implement stpcpy
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LLVM implemented a recent "libcall optimization" that lowers calls to
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`sprintf(dest, "%s", str)` where the return value is used to `stpcpy(dest,
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str) - dest`. This generally avoids the machinery involved in parsing
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format strings. `stpcpy` is just like `strcpy` except it returns the
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pointer to the new tail of `dest`. This optimization was introduced into
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clang-12.
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Implement this so that we don't observe linkage failures due to missing
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symbol definitions for `stpcpy`.
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Similar to last year's fire drill with: commit 5f074f3e192f
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("lib/string.c: implement a basic bcmp")
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The kernel is somewhere between a "freestanding" environment (no full
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libc) and "hosted" environment (many symbols from libc exist with the same
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type, function signature, and semantics).
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As H. Peter Anvin notes, there's not really a great way to inform the
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compiler that you're targeting a freestanding environment but would like
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to opt-in to some libcall optimizations (see pr/47280 below), rather than
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opt-out.
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Arvind notes, -fno-builtin-* behaves slightly differently between GCC and
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Clang, and Clang is missing many __builtin_* definitions, which I consider
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a bug in Clang and am working on fixing.
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Masahiro summarizes the subtle distinction between compilers justly:
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To prevent transformation from foo() into bar(), there are two ways in
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Clang to do that; -fno-builtin-foo, and -fno-builtin-bar. There is
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only one in GCC; -fno-buitin-foo.
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(Any difference in that behavior in Clang is likely a bug from a missing
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__builtin_* definition.)
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Masahiro also notes:
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We want to disable optimization from foo() to bar(),
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but we may still benefit from the optimization from
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foo() into something else. If GCC implements the same transform, we
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would run into a problem because it is not -fno-builtin-bar, but
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-fno-builtin-foo that disables that optimization.
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In this regard, -fno-builtin-foo would be more future-proof than
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-fno-built-bar, but -fno-builtin-foo is still potentially overkill. We
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may want to prevent calls from foo() being optimized into calls to
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bar(), but we still may want other optimization on calls to foo().
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It seems that compilers today don't quite provide the fine grain control
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over which libcall optimizations pseudo-freestanding environments would
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prefer.
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Finally, Kees notes that this interface is unsafe, so we should not
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encourage its use. As such, I've removed the declaration from any header,
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but it still needs to be exported to avoid linkage errors in modules.
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Link: https://lkml.kernel.org/r/20200825140001.2941001-1-ndesaulniers@google.com
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Link: https://bugs.llvm.org/show_bug.cgi?id=47162
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Link: https://bugs.llvm.org/show_bug.cgi?id=47280
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Link: https://github.com/ClangBuiltLinux/linux/issues/1126
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Link: https://man7.org/linux/man-pages/man3/stpcpy.3.html
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Link: https://pubs.opengroup.org/onlinepubs/9699919799/functions/stpcpy.html
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Link: https://reviews.llvm.org/D85963
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Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
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Reported-by: Sami Tolvanen <samitolvanen@google.com>
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Tested-by: Nathan Chancellor <natechancellor@gmail.com>
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Suggested-by: Andy Lavr <andy.lavr@gmail.com>
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Suggested-by: Arvind Sankar <nivedita@alum.mit.edu>
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Suggested-by: Joe Perches <joe@perches.com>
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Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
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Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
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Cc: <stable@vger.kernel.org>
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Cc: Kees Cook <keescook@chromium.org>
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Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
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Cc: Alexandru Ardelean <alexandru.ardelean@analog.com>
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Cc: Yury Norov <yury.norov@gmail.com>
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Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
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---
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lib/string.c | 24 ++++++++++++++++++++++++
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1 file changed, 24 insertions(+)
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diff --git a/lib/string.c b/lib/string.c
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index 6012c385fb314d..6bd0cf0fb009a2 100644
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--- a/lib/string.c
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+++ b/lib/string.c
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@@ -272,6 +272,30 @@ ssize_t strscpy_pad(char *dest, const char *src, size_t count)
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}
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EXPORT_SYMBOL(strscpy_pad);
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+/**
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+ * stpcpy - copy a string from src to dest returning a pointer to the new end
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+ * of dest, including src's %NUL-terminator. May overrun dest.
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+ * @dest: pointer to end of string being copied into. Must be large enough
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+ * to receive copy.
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+ * @src: pointer to the beginning of string being copied from. Must not overlap
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+ * dest.
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+ *
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+ * stpcpy differs from strcpy in a key way: the return value is the new
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+ * %NUL-terminated character. (for strcpy, the return value is a pointer to
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+ * src. This interface is considered unsafe as it doesn't perform bounds
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+ * checking of the inputs. As such it's not recommended for usage. Instead,
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+ * its definition is provided in case the compiler lowers other libcalls to
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+ * stpcpy.
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+ */
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+char *stpcpy(char *__restrict__ dest, const char *__restrict__ src);
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+char *stpcpy(char *__restrict__ dest, const char *__restrict__ src)
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+{
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+ while ((*dest++ = *src++) != '\0')
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+ /* nothing */;
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+ return --dest;
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+}
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+EXPORT_SYMBOL(stpcpy);
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+
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#ifndef __HAVE_ARCH_STRCAT
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/**
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* strcat - Append one %NUL-terminated string to another
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--
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cgit 1.2.3-korg
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