Re: [PATCH] nptl: Add compiler barrier in nptl/tst-pthread-getattr

[Florian Weimer <fweimer at redhat dot com>]

* Andreas Schwab:

> On Jul 22 2019, Florian Weimer <fweimer@redhat.com> wrote:
>
>> Recent GCC versions warn about the attempt to return the address of a
>> local variable:
>>
>> tst-pthread-getattr.c: In function ‘allocate_and_test’:
>> tst-pthread-getattr.c:54:10: error: function returns address of local variable [-Werror=return-local-addr]
>>    54 |   return mem;
>>       |          ^~~
>> In file included from ../include/alloca.h:3,
>>                  from tst-pthread-getattr.c:26:
>> ../stdlib/alloca.h:35:23: note: declared here
>>    35 | # define alloca(size) __builtin_alloca (size)
>>       |                       ^~~~~~~~~~~~~~~~~~~~~~~
>> tst-pthread-getattr.c:51:9: note: in expansion of macro ‘alloca’
>>    51 |   mem = alloca ((size_t) (mem - target));
>>       |         ^~~~~~
>>
>> 2019-07-22  Florian Weimer  <fweimer@redhat.com>
>>
>> 	* nptl/tst-pthread-getattr.c (compiler_barrier): New function.
>> 	(allocate_and_test): Use it.
>
> Does it work to change the return type to uintptr_t?

Thanks for the suggestion.  I think this is the better fix.  Patch
below.

Florian

nptl: Use uintptr_t for address diagnostic in nptl/tst-pthread-getattr

Recent GCC versions warn about the attempt to return the address of a
local variable:

tst-pthread-getattr.c: In function ‘allocate_and_test’:
tst-pthread-getattr.c:54:10: error: function returns address of local variable [-Werror=return-local-addr]
   54 |   return mem;
      |          ^~~
In file included from ../include/alloca.h:3,
                 from tst-pthread-getattr.c:26:
../stdlib/alloca.h:35:23: note: declared here
   35 | # define alloca(size) __builtin_alloca (size)
      |                       ^~~~~~~~~~~~~~~~~~~~~~~
tst-pthread-getattr.c:51:9: note: in expansion of macro ‘alloca’
   51 |   mem = alloca ((size_t) (mem - target));
      |         ^~~~~~

The address itself is used in a check in the caller, so using
uintptr_t instead is reasonable.

2019-07-22  Florian Weimer  <fweimer@redhat.com>

	* nptl/tst-pthread-getattr.c (allocate_and_test): Change return
	type to uintptr_t.
	(check_stack_top): Adjust.

diff --git a/nptl/tst-pthread-getattr.c b/nptl/tst-pthread-getattr.c
index a954778767..e3634ea0a7 100644
--- a/nptl/tst-pthread-getattr.c
+++ b/nptl/tst-pthread-getattr.c
@@ -43,7 +43,7 @@ static size_t pagesize;
 
 /* Check if the page in which TARGET lies is accessible.  This will segfault
    if it fails.  */
-static volatile char *
+static volatile uintptr_t
 allocate_and_test (char *target)
 {
   volatile char *mem = (char *) &mem;
@@ -51,7 +51,7 @@ allocate_and_test (char *target)
   mem = alloca ((size_t) (mem - target));
 
   *mem = 42;
-  return mem;
+  return (uintptr_t) mem;
 }
 
 static int
@@ -84,7 +84,6 @@ check_stack_top (void)
 {
   struct rlimit stack_limit;
   void *stackaddr;
-  volatile void *mem;
   size_t stacksize = 0;
   int ret;
   uintptr_t pagemask = ~(pagesize - 1);
@@ -130,14 +129,14 @@ check_stack_top (void)
      stack and test access there.  It is however sufficient to simply check if
      the top page is accessible, so we target our access halfway up the top
      page.  Thanks Chris Metcalf for this idea.  */
-  mem = allocate_and_test (stackaddr + pagesize / 2);
+  uintptr_t mem = allocate_and_test (stackaddr + pagesize / 2);
 
   /* Before we celebrate, make sure we actually did test the same page.  */
-  if (((uintptr_t) stackaddr & pagemask) != ((uintptr_t) mem & pagemask))
+  if (((uintptr_t) stackaddr & pagemask) != (mem & pagemask))
     {
       printf ("We successfully wrote into the wrong page.\n"
 	      "Expected %#" PRIxPTR ", but got %#" PRIxPTR "\n",
-	      (uintptr_t) stackaddr & pagemask, (uintptr_t) mem & pagemask);
+	      (uintptr_t) stackaddr & pagemask, mem & pagemask);
 
       return 1;
     }