linux-kernel - [PATCH nolibc 52/61] tools/nolibc/stdlib: only reference the external environ when inlined

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Message-Id: <20220419004225.3952530-52-paulmck@kernel.org>
Date:   Mon, 18 Apr 2022 17:42:16 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     linux-kernel@...r.kernel.org
Cc:     gwml@...r.gnuweeb.org, kernel-team@...com, w@....eu,
        Willy Tarreau <w@....eu>,
        Ammar Faizi <ammarfaizi2@...weeb.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH nolibc 52/61] tools/nolibc/stdlib: only reference the external environ when inlined

From: Willy Tarreau <w@....eu>

When building with gcc at -O0 we're seeing link errors due to the
"environ" variable being referenced by getenv(). The problem is that
at -O0 gcc will not inline getenv() and will not drop the external
reference. One solution would be to locally declare the variable as
weak, but then it would appear in all programs even those not using
it, and would be confusing to users of getenv() who would forget to
set environ to envp.

An alternate approach used in this patch consists in always inlining
the outer part of getenv() that references this extern so that it's
always dropped when not used. The biggest part of the function was
now moved to a new function called _getenv() that's still not inlined
by default.

Reported-by: Ammar Faizi <ammarfaizi2@...weeb.org>
Signed-off-by: Willy Tarreau <w@....eu>
Tested-by: Ammar Faizi <ammarfaizi2@...weeb.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 tools/include/nolibc/stdlib.h | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/tools/include/nolibc/stdlib.h b/tools/include/nolibc/stdlib.h
index aca8616335e3..8a07e263f0d0 100644
--- a/tools/include/nolibc/stdlib.h
+++ b/tools/include/nolibc/stdlib.h
@@ -60,16 +60,17 @@ int atoi(const char *s)
 	return atol(s);
 }
 
-/* Tries to find the environment variable named <name> in the environment array
- * pointed to by global variable "environ" which must be declared as a char **,
- * and must be terminated by a NULL (it is recommended to set this variable to
- * the "envp" argument of main()). If the requested environment variable exists
- * its value is returned otherwise NULL is returned.
+/* getenv() tries to find the environment variable named <name> in the
+ * environment array pointed to by global variable "environ" which must be
+ * declared as a char **, and must be terminated by a NULL (it is recommended
+ * to set this variable to the "envp" argument of main()). If the requested
+ * environment variable exists its value is returned otherwise NULL is
+ * returned. getenv() is forcefully inlined so that the reference to "environ"
+ * will be dropped if unused, even at -O0.
  */
 static __attribute__((unused))
-char *getenv(const char *name)
+char *_getenv(const char *name, char **environ)
 {
-	extern char **environ;
 	int idx, i;
 
 	if (environ) {
@@ -83,6 +84,13 @@ char *getenv(const char *name)
 	return NULL;
 }
 
+static inline __attribute__((unused,always_inline))
+char *getenv(const char *name)
+{
+	extern char **environ;
+	return _getenv(name, environ);
+}
+
 /* Converts the unsigned long integer <in> to its hex representation into
  * buffer <buffer>, which must be long enough to store the number and the
  * trailing zero (17 bytes for "ffffffffffffffff" or 9 for "ffffffff"). The
-- 
2.31.1.189.g2e36527f23