[PATCH] lib/string.c: implement stpcpy

[Nick Desaulniers <ndesaulniers@google.com>]

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:
1. it returns the pointer to the new tail of `dest`. This allows you to
   chain multiple calls to `stpcpy` in one statement.
2. it requires the parameters not to overlap.  Calling `sprintf` with
   overlapping arguments was clarified in ISO C99 and POSIX.1-2001 to be
   undefined behavior.

`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
Reported-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
 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..e570b9b10f50 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..81bc4d62c256 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 NULL 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