Kees Cook wrote:
> > Consider, e.g. introducing __vsnprint(), with vsnprintf(s, n, fmt, ...)
> > expanding to __vsnprintf(1, s, n, fmt, ...) if fmt is a string literal
> > and __vsnprintf(0, s, n, fmt, ...) otherwise. Now,
> > int __sprintf(int safe, char *buf, const char *fmt, ...)
> > {
> > va_list args;
> > int i;
> >
> > va_start(args, fmt);
> > i = __vsnprintf(safe, buf, INT_MAX, fmt, args);
> > va_end(args);
> >
> > return i;
> > }
>
> Unless I've misunderstood, I think we'd already get very close to this
> with the gcc options instead. This patch set is what I've been using
> to generate the format string fixes over the last few months, with 7
> sent this last round:
Can we utilize __builtin_constant_p() ?
---------- source start ----------
#include <stdio.h>
#define func(fmt) \
if (__builtin_constant_p(fmt)) \
printf("const : %s\n", fmt); \
else \
printf("not const : %s\n", fmt); \
int main(int argc, char *argv[])
{
const char *fmt1 = "const char *";
const char fmt2[] = "const char []";
const char * const fmt3 = "const char * const";
func("literal");
func(fmt1);
func(fmt2);
func(fmt3);
return 0;
}
---------- source end ----------
---------- output start ----------
const : literal
not const : const char *
not const : const char []
const : const char * const
---------- output end ----------
__builtin_constant_p() seems to enforce use of either "literal" or "* const".
An example change
---------- patch start ----------
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -120,8 +120,9 @@ asmlinkage int printk_emit(int facility, int level,
const char *dict, size_t dictlen,
const char *fmt, ...);
-asmlinkage __printf(1, 2) __cold
-int printk(const char *fmt, ...);
+//asmlinkage __printf(1, 2) __cold
+//int printk(const char *fmt, ...);
+#define printk(fmt, ...) compiletime_assert(__builtin_constant_p(fmt), "Non-c
onstant format string")
/*
* Special printk facility for scheduler use only, _DO_NOT_USE_ !
---------- patch end ----------
caught errors like below.
CC [M] drivers/scsi/esas2r/esas2r_log.o
drivers/scsi/esas2r/esas2r_log.c: In function 'esas2r_log_master':
drivers/scsi/esas2r/esas2r_log.c:174: error: call to '__compiletime_assert_174'
declared with attribute error: Non-constant format string
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