Messages in this thread | | | Date | Sun, 20 Feb 2011 19:08:45 +0100 | From | Borislav Petkov <> | Subject | Re: [patch -next] x86, microcode, AMD: signedness bug in generic_load_microcode() |
| |
On Sun, Feb 20, 2011 at 10:50:11AM -0700, Matthew Wilcox wrote: > On Sun, Feb 20, 2011 at 03:14:52PM +0100, Borislav Petkov wrote: > > int f() { > > return 0xa5a5a5a5; > > } > > > > int main() > > { > > > > char ret = f(); > > > > printf("ret = 0x%016x\n", ret); > > > > return 0; > > } > > -- > > > > doesn't cause a warning and prints a sign extended 0x00000000ffffffa5 > > which is cast to the return type of the function. If ret is an unsigned > > char, then we return a 0x00000000000000a5. > > > > I found something about it in the C99 standard??, section "6.5.16.1 Simple > > assignment": > > > > 4. EXAMPLE 1 In the program fragment > > > > int f(void); > > char c; > > /* ... */ > > if ((c = f()) == -1) > > /* ... */ > > > > the int value returned by the function may be truncated when stored in > > the char, and then converted back to int width prior to the comparison. > > In an implementation in which ??????plain?????? char has the same range > > of values as unsigned char (and char is narrower than int), the result > > of the conversion cannot be negative, so the operands of the comparison > > can never compare equal. Therefore, for full portability, the variable c > > should be declared as int." > > > > so the whole "... may be truncated.. " could mean a lot of things. From > > my example above, gcc does truncate the int return type to a byte-sized > > char only when they differ in signedness. > > No, that's not what's going on. GCC _is_ truncating to a byte, 0xa5, > whether it's signed or not. Then at the time of the call to printf, > the 0xa5 is cast to int. If the char is signed, 0xa5 is sign-extended; > if unsigned, it's zero-extended.
Yes, you're right, I missed the fact that printf does convert its arguments based on the format string. I should've done
printf("ret = 0x%hhx\n", ret);
for chars.
Thanks.
-- Regards/Gruss, Boris.
| |