[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20110220175011.GA13726@parisc-linux.org>
Date: Sun, 20 Feb 2011 10:50:11 -0700
From: Matthew Wilcox <matthew@....cx>
To: Borislav Petkov <bp@...64.org>
Cc: Ingo Molnar <mingo@...e.hu>, Dan Carpenter <error27@...il.com>,
"Herrmann3, Andreas" <Andreas.Herrmann3@....com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
"H. Peter Anvin" <hpa@...or.com>,
"maintainer:X86 ARCHITECTURE..." <x86@...nel.org>,
"open list:AMD MICROCODE UPD..." <amd64-microcode@...64.org>,
open list <linux-kernel@...r.kernel.org>,
"kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>
Subject: Re: [patch -next] x86, microcode, AMD: signedness bug in
generic_load_microcode()
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.
--
Matthew Wilcox Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours. We can't possibly take such
a retrograde step."
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists