undefined symbol: stpcpy

[nickdesaulniers]

@samitolvanen reported upstream that 6dbf0cfcf789365493f70ae69df8a7a59be41c75 is breaking builds by lowering calls sprintf(dst, "%s", str) -> strcpy(dst, str)

patch:

From e1a2727a16d8b417ed5ba70c12642741b544e80c Mon Sep 17 00:00:00 2001
From: Nick Desaulniers <ndesaulniers@google.com>
Date: Fri, 14 Aug 2020 16:55:58 -0700
Subject: [PATCH] lib/string.c: implement stpcpy

The tricksters over at LLVM implemented a recent "libcall optimization"
that lowers calls to `sprintf(dst, "%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")

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
Reported-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
 include/linux/string.h |  3 +++
 lib/string.c           | 21 +++++++++++++++++++++
 2 files changed, 24 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..04515869652d 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -272,6 +272,28 @@ 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.
+ * @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');
+       return dest;
+}
+#endif
+

[nickdesaulniers]

sent: https://lore.kernel.org/lkml/20200815002417.1512973-1-ndesaulniers@google.com/T/#u

[AndyLavr]

Fix:

diff --git a/lib/string.c b/lib/string.c
--- a/lib/string.c
+++ b/lib/string.c
@@ -293,6 +293,7 @@ char *stpcpy(char *__restrict__ dest, const char *__restrict__ src)
 		/* nothing */;
 	return dest;
 }
+EXPORT_SYMBOL(stpcpy);
 #endif
 
 #ifndef __HAVE_ARCH_STRCAT

[nickdesaulniers]

https://lore.kernel.org/lkml/20200819191654.1130563-1-ndesaulniers@google.com/T/#t