From: Nick Desaulniers <ndesaulniers@google.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: "Dávid Bolvanský" <david.bolvansky@gmail.com>,
"Eli Friedman" <efriedma@quicinc.com>,
"Nick Desaulniers" <ndesaulniers@google.com>,
stable@vger.kernel.org, "Arvind Sankar" <nivedita@alum.mit.edu>,
"Joe Perches" <joe@perches.com>,
"Sami Tolvanen" <samitolvanen@google.com>,
"Vishal Verma" <vishal.l.verma@intel.com>,
"Dan Williams" <dan.j.williams@intel.com>,
"Andy Shevchenko" <andriy.shevchenko@linux.intel.com>,
"Joel Fernandes (Google)" <joel@joelfernandes.org>,
"Daniel Axtens" <dja@axtens.net>,
"Kees Cook" <keescook@chromium.org>,
"Ingo Molnar" <mingo@kernel.org>,
"Yury Norov" <yury.norov@gmail.com>,
"Alexandru Ardelean" <alexandru.ardelean@analog.com>,
linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com
Subject: [PATCH v2] lib/string.c: implement stpcpy
Date: Fri, 14 Aug 2020 19:09:44 -0700 [thread overview]
Message-ID: <20200815020946.1538085-1-ndesaulniers@google.com> (raw)
In-Reply-To: <20200815014006.GB99152@rani.riverdale.lan>
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.
`stpcpy` was first standardized in POSIX.1-2008.
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")
This optimization was introduced into clang-12.
Cc: stable@vger.kernel.org
Link: https://bugs.llvm.org/show_bug.cgi?id=47162
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
Suggested-by: Arvind Sankar <nivedita@alum.mit.edu>
Suggested-by: Joe Perches <joe@perches.com>
Reported-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
Changes V2:
* Added Sami's Tested by; though the patch changed implementation, the
missing symbol at link time was the problem Sami was observing.
* Fix __restrict -> __restrict__ typo as per Joe.
* Drop note about restrict from commit message as per Arvind.
* Fix NULL -> NUL as per Arvind; NUL is ASCII '\0'. TIL
* Fix off by one error as per Arvind; I had another off by one error in
my test program that was masking this.
include/linux/string.h | 3 +++
lib/string.c | 23 +++++++++++++++++++++++
2 files changed, 26 insertions(+)
diff --git a/include/linux/string.h b/include/linux/string.h
index b1f3894a0a3e..7686dbca8582 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -31,6 +31,9 @@ size_t strlcpy(char *, const char *, size_t);
#ifndef __HAVE_ARCH_STRSCPY
ssize_t strscpy(char *, const char *, size_t);
#endif
+#ifndef __HAVE_ARCH_STPCPY
+extern char *stpcpy(char *__restrict__, const char *__restrict__);
+#endif
/* Wraps calls to strscpy()/memset(), no arch specific code required */
ssize_t strscpy_pad(char *dest, const char *src, size_t count);
diff --git a/lib/string.c b/lib/string.c
index 6012c385fb31..68ddbffbbd58 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -272,6 +272,29 @@ ssize_t strscpy_pad(char *dest, const char *src, size_t count)
}
EXPORT_SYMBOL(strscpy_pad);
+#ifndef __HAVE_ARCH_STPCPY
+/**
+ * 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 two key ways:
+ * 1. inputs must not overlap.
+ * 2. return value is the new NULL terminated character. (for strcpy, the
+ * return value is a pointer to src.
+ */
+#undef stpcpy
+char *stpcpy(char *__restrict__ dest, const char *__restrict__ src)
+{
+ while ((*dest++ = *src++) != '\0')
+ /* nothing */;
+ return --dest;
+}
+#endif
+
#ifndef __HAVE_ARCH_STRCAT
/**
* strcat - Append one %NUL-terminated string to another
--
2.28.0.220.ged08abb693-goog
next prev parent reply other threads:[~2020-08-15 21:55 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-15 0:24 [PATCH] lib/string.c: implement stpcpy Nick Desaulniers
2020-08-15 0:52 ` Joe Perches
2020-08-15 2:00 ` Nick Desaulniers
2020-08-15 0:53 ` Sami Tolvanen
2020-08-15 1:33 ` Arvind Sankar
2020-08-15 1:40 ` Arvind Sankar
2020-08-15 2:09 ` Nick Desaulniers [this message]
2020-08-15 2:58 ` [PATCH v2] " Arvind Sankar
2020-08-15 3:42 ` Joe Perches
2020-08-15 16:34 ` Kees Cook
2020-08-15 17:38 ` Dávid Bolvanský
2020-08-15 20:47 ` Nick Desaulniers
2020-08-15 21:23 ` Joe Perches
2020-08-15 21:27 ` Dávid Bolvanský
2020-08-15 21:28 ` Nick Desaulniers
2020-08-15 21:31 ` Joe Perches
2020-08-15 22:17 ` Nick Desaulniers
2020-08-16 0:19 ` Fangrui Song
2020-08-16 5:22 ` Sedat Dilek
2020-08-16 15:02 ` Arvind Sankar
2020-08-17 17:14 ` Sami Tolvanen
2020-08-17 18:36 ` Nick Desaulniers
2020-08-17 19:15 ` Kees Cook
2020-08-17 20:13 ` Arvind Sankar
2020-08-17 21:45 ` Nick Desaulniers
[not found] ` <77557c29286140dea726cc334b4f59fc@AcuMS.aculab.com>
2020-08-18 8:32 ` Joe Perches
2020-08-17 19:16 ` Kees Cook
2020-08-15 2:00 ` [PATCH] " Nick Desaulniers
2020-08-15 22:17 ` David Laight
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