Hi,
while reading some interesting stuff about memset being optimized
away by compilers if the variable is not read after the memset call
I recalled there was something similar in libbb/obscure.c file:
static int string_checker(const char *p1, const char *p2)
{
int size, i;
/* check string */
int ret = string_checker_helper(p1, p2);
/* make our own copy */
char *p = xstrdup(p1);
/* reverse string */
i = size = strlen(p1);
while (--i >= 0) {
*p++ = p1[i];
}
p -= size; /* restore pointer */
/* check reversed string */
ret |= string_checker_helper(p, p2);
/* clean up */
memset(p, 0, size);
free(p);
return ret;
}
I've tried to find out if memset is really optimized away in this case
with some test code that I've compiled with :
gcc -O0 -S test.c
and then with -O1 -O2 -O3 -Os.
At a first glance the memset call is there only with the -O0 flag.
I've then tried the libbb nuke_str call instead ans it seems to
survive compiler optimization, therefore I propose the following
patch to obscure.c:
--- libbb/obscure.c.orig 2013-06-02 13:56:34.000000000 +0200
+++ libbb/obscure.c 2014-04-16 18:20:39.783664194 +0200
@@ -75,8 +75,9 @@
/* check reversed string */
ret |= string_checker_helper(p, p2);
- /* clean up */
- memset(p, 0, size);
+ /* clean up, don't use memset as it is optimized away by compiler */
+ /*memset(p, 0, size);*/
+ nuke_str(p);
free(p);
return ret;
As my understanding of the assembler code generated with gcc -S
is _VERY_ limited and due to the fact that it was tested on a different
handcrafted test file I'm not 100% sure the same happens in busybox
but I suspect it so more experienced programmers should take a look
at it.
Ciao,
Tito
Use nuke_str as memeset is optimized away by compiler.
Signed-off-by: Tito Ragusa <[email protected]>
--- libbb/obscure.c.orig 2013-06-02 13:56:34.000000000 +0200
+++ libbb/obscure.c 2014-04-16 18:20:39.783664194 +0200
@@ -75,8 +75,9 @@
/* check reversed string */
ret |= string_checker_helper(p, p2);
- /* clean up */
- memset(p, 0, size);
+ /* clean up, don't use memset as it is optimized away by compiler */
+ /*memset(p, 0, size);*/
+ nuke_str(p);
free(p);
return ret;
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