Re: [PATCH 1/2] explicit_bzero-tests: pacify GCC

[Paul Eggert <eggert@cs.ucla.edu>]

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On 7/18/21 4:12 AM, Bruno Haible wrote:
> The purpose of the test is to verify that the compiler does not eliminate
> a call to explicit_bzero, even if data flow analysis reveals that the stack
> area is "dead" at the end of the function.
>
> With this patch, it was turned into a weaker assertion: namely, that the
> compiler does not eliminate a call to explicit_bzero if it cannot make
> inferences about the pointer argument.

Strictly speaking, neither assertion is weaker than the other. However, 
I take your point that the patch changes the meaning of the test in an 
undesirable way. I installed the attached to implement your suggestion.

This new diagnostic points out a problem with the test, though. If GCC 
can determine that memcmp reads uninitialized storage, GCC can optimize 
away the memcmp and act as if memcmp returns 0 (or any other constant). 
So the test as it stands is problematic given recent advances in 
practical compilers.

As an aside, I don't understand the discussion of asynchronous signal 
invocations in that test's commentary. There is no asynchronous 
signaling in that code.

(These points are of course low-priority, as explicit_bzero is 
documented to be best-effort as opposed to being a guarantee that the 
information is erased.)



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From f9803478355d038aa060d71bdd9eddf2bd43325f Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Sun, 18 Jul 2021 14:08:56 -0500
Subject: [PATCH] explicit_bzero-tests: pacify GCC better

Problem reported by Bruno Haible in:
https://lists.gnu.org/r/bug-gnulib/2021-07/msg00039.html
* tests/test-explicit_bzero.c: Ignore -Wmaybe-uninitialized.
(stackbuf): Remove this static pointer, reverting recent change.
(do_secret_stuff, test_stack): Revert these related changes too.
---
 ChangeLog                   |  9 +++++++++
 tests/test-explicit_bzero.c | 16 +++++++++++-----
 2 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index c50808437..d175c39af 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+2021-07-18  Paul Eggert  <eggert@cs.ucla.edu>
+
+	explicit_bzero-tests: pacify GCC better
+	Problem reported by Bruno Haible in:
+	https://lists.gnu.org/r/bug-gnulib/2021-07/msg00039.html
+	* tests/test-explicit_bzero.c: Ignore -Wmaybe-uninitialized.
+	(stackbuf): Remove this static pointer, reverting recent change.
+	(do_secret_stuff, test_stack): Revert these related changes too.
+
 2021-07-17  Paul Eggert  <eggert@cs.ucla.edu>
 
 	memrchr-tests: pacify GCC
diff --git a/tests/test-explicit_bzero.c b/tests/test-explicit_bzero.c
index c42aba93f..14f0ead2b 100644
--- a/tests/test-explicit_bzero.c
+++ b/tests/test-explicit_bzero.c
@@ -32,6 +32,12 @@ SIGNATURE_CHECK (explicit_bzero, void, (void *, size_t));
 #include "vma-iter.h"
 #include "macros.h"
 
+/* Suppress GCC warning that do_secret_stuff (2) reads uninitialized
+   local storage.  */
+#if 4 < __GNUC__ + (3 <= __GNUC_MINOR__)
+# pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
+
 #define SECRET "xyzzy1729"
 #define SECRET_SIZE 9
 
@@ -126,12 +132,14 @@ test_heap (void)
 /* There are two passes:
      1. Put a secret in memory and invoke explicit_bzero on it.
      2. Verify that the memory has been erased.
-   Access the memory via a volatile pointer, so the compiler
-   does not assume the pointer's value and optimize away accesses.  */
-static char *volatile stackbuf;
+   Implement them in the same function, so that they access the same memory
+   range on the stack.  That way, the test verifies that the compiler
+   does not eliminate a call to explicit_bzero, even if data flow analysis
+   reveals that the stack area is dead at the end of the function.  */
 static int _GL_ATTRIBUTE_NOINLINE
 do_secret_stuff (volatile int pass)
 {
+  char stackbuf[SECRET_SIZE];
   if (pass == 1)
     {
       memcpy (stackbuf, SECRET, SECRET_SIZE);
@@ -147,8 +155,6 @@ do_secret_stuff (volatile int pass)
 static void
 test_stack (void)
 {
-  char stack_buffer[SECRET_SIZE];
-  stackbuf = stack_buffer;
   int count = 0;
   int repeat;
 
-- 
2.25.1