Messages in this thread | | | From | roel kluin <> | Date | Thu, 25 Feb 2010 16:31:36 +0100 | Subject | Re: [PATCH] sha: prevent removal of memset as dead store in sha1_update() |
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On Thu, Feb 25, 2010 at 4:17 PM, David Miller <davem@davemloft.net> wrote: > From: Roel Kluin <roel.kluin@gmail.com> > Date: Thu, 25 Feb 2010 16:10:27 +0100 > >> Due to optimization A call to memset() may be removed as a dead store when >> the buffer is not used after its value is overwritten. >> >> Signed-off-by: Roel Kluin <roel.kluin@gmail.com> > > Solution is wrong and overkill in my mind. > > It's overkill because the whole reason it's using a stack buffer is to > avoid the overhead of a kmalloc() call. > > And it's wrong because the reason the memset() is there seems to be > to clear out key information that might exist kernel stack so that > it's more difficult for rogue code to get at things.
If the memset is optimized away then the clear out does not occur. Do you know a different way to fix this? I observed this with:
$ gcc -O2 test.c;./a.out and It shows (on my box) "...S.e.c.r.e.t..."
$ cat test.c
#include <stdio.h> #include <string.h> #include <stdlib.h>
#define ON_STACK 1
void foo() { char password[] = "secret"; password[0]='S'; printf ("Don't show again: %s\n", password); memset(password, 0, sizeof(password)); }
void foo2() { char* password = malloc(7); strncpy (password, "secret" , 7); password[6] = '\0'; password[0] = 'S'; printf ("Don't show again: %s\n", password); //memset(password, 0, 7); free(password);
}
int main(int argc, char* argv[]) {
#if ON_STACK == 1 foo(); #else foo2(); #endif int i; char foo3[] = "hoi"; printf ("foo1:%s\n", foo3); char* bar = &foo3[0]; for (i = -50; i < 50; i++) printf ("%c.", bar[i]); printf("\n"); return 0; }
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