[PATCH][BZ 15846][v2] Add error reporting (via errno) to getauxval().

[Brooks Moses <bmoses at google dot com>]

Ok, since the general approach seems plausible, here's a real patch.

As discussed in the recent thread on my $EXEC_ORIGIN patch and in BZ 
15846, getauxval() presently has no unambiguous way of reporting an 
error condition.  It currently returns zero on error, but this may also 
be a valid result for some auxv entries.  As there is no clear invalid 
result for all current and future auxv entries, this patch sets errno 
(following a suggestion in the BZ entry).

This version of the patch also adds documentation and tests for the 
value-not-found conditions in getauxval().  Tested on x86_64 GNU/Linux.

(Adding tests showed me that the patch was incomplete; it's no longer a 
"const" function because it has a side effect of changing errno, and 
until I removed __attribute_const__ from the declaration, GCC simply 
discarded the call to it in the test!)

Ok to commit?

---
2013-12-10  Brooks Moses  <bmoses@google.com>

	[BZ #15846]
	* misc/getauxval.c: Include errno.h.
	(__getauxval): Set errno to ENOENT if the requested type is not
	found.
	* misc/sys/auxv.h (getauxval): Document that it may set errno;
	don't declare with __attribute_const__.
	* elf/tst-auxv.c: Add tests for errno and type-not-found case.
	* manual/startup.texi: Document that getauxval sets errno.

 elf/tst-auxv.c      | 26 +++++++++++++++++++++++++-
 manual/startup.texi |  3 ++-
 misc/getauxval.c    |  3 +++
 misc/sys/auxv.h     |  4 ++--
 4 files changed, 32 insertions(+), 4 deletions(-)

diff --git a/elf/tst-auxv.c b/elf/tst-auxv.c
index 454c0b0..3ed4380 100644
--- a/elf/tst-auxv.c
+++ b/elf/tst-auxv.c
@@ -16,6 +16,7 @@
    <http://www.gnu.org/licenses/>.  */
 
 #include <elf.h>
+#include <errno.h>
 #include <link.h>
 #include <string.h>
 #include <stdio.h>
@@ -25,14 +26,37 @@
 static int
 do_test (int argc, char *argv[])
 {
+  errno = 0;
+  const char *execfn = (const char *) getauxval (AT_NULL);
+
+  if (errno != ENOENT)
+    {
+      printf ("errno is %d rather than %d (ENOENT) on failure\n", errno,
+	      ENOENT);
+      return 1;
+    }
+
+  if (execfn != NULL)
+    {
+      printf ("getauxval return value is nonzero on failure\n");
+      return 1;
+    }
+
+  errno = 0;
   const char *execfn = (const char *) getauxval (AT_EXECFN);
 
   if (execfn == NULL)
     {
-      printf ("No AT_EXECFN found, test skipped\n");
+      printf ("No AT_EXECFN found, AT_EXECFN test skipped\n");
       return 0;
     }
 
+  if (errno != 0)
+    {
+      printf ("errno erroneously set to %d on success\n", errno);
+      return 1;
+    }
+
   if (strcmp (argv[0], execfn) != 0)
     {
       printf ("Mismatch: argv[0]: %s vs. AT_EXECFN: %s\n", argv[0], execfn);
diff --git a/manual/startup.texi b/manual/startup.texi
index a277714..edd1de4 100644
--- a/manual/startup.texi
+++ b/manual/startup.texi
@@ -625,7 +625,8 @@ basis there may be information that is not available any other way.
 This function is used to inquire about the entries in the auxiliary
 vector.  The @var{type} argument should be one of the @samp{AT_} symbols
 defined in @file{elf.h}.  If a matching entry is found, the value is
-returned; if the entry is not found, zero is returned.
+returned; if the entry is not found, zero is returned and @code{errno} is
+set to @code{ENOENT}.
 @end deftypefun
 
 For some platforms, the key @code{AT_HWCAP} is the easiest way to inquire
diff --git a/misc/getauxval.c b/misc/getauxval.c
index e0317fd..dd4c8ec 100644
--- a/misc/getauxval.c
+++ b/misc/getauxval.c
@@ -16,6 +16,7 @@
    <http://www.gnu.org/licenses/>.  */
 
 #include <sys/auxv.h>
+#include <errno.h>
 #include <ldsodefs.h>
 
 
@@ -32,6 +33,8 @@ __getauxval (unsigned long int type)
   for (p = GLRO(dl_auxv); p->a_type != AT_NULL; p++)
     if (p->a_type == type)
       return p->a_un.a_val;
+
+  __set_errno (ENOENT);
   return 0;
 }
 
diff --git a/misc/sys/auxv.h b/misc/sys/auxv.h
index a69250b..7aec3a0 100644
--- a/misc/sys/auxv.h
+++ b/misc/sys/auxv.h
@@ -27,9 +27,9 @@ __BEGIN_DECLS
 
 /* Return the value associated with an Elf*_auxv_t type from the auxv list
    passed to the program on startup.  If TYPE was not present in the auxv
-   list, returns zero.  */
+   list, returns zero and sets errno to ENOENT.  */
 extern unsigned long int getauxval (unsigned long int __type)
-  __THROW __attribute_const__;
+  __THROW;
 
 __END_DECLS
 
-- 
1.8.5.1