fix broken overflow check in digest_certinfo()

gcc compiles `p + length < p' into 'length < 0' since pointer overflow is undefined behavior in C.  This breaks the check against a large `length'. 

Use `length > pend - p' instead.

There's no need to check `length < 0' since `length' is from length_decode() and should be non-negative. 

===

Try the simplified code.

void bar(void);
void foo(unsigned char *p, int length)
{
	if (p + length < p)
		bar();
}

$ gcc -S -o - t.c -O2
...
foo:
.LFB0:
	.cfi_startproc
	testl	%esi, %esi
	js	.L4
	rep
	ret
.L4:
	jmp	bar
	.cfi_endproc

Clearly `p' is not used at all.

[rsc, 2013-01-31 15:50:59]

I don't understand what's wrong here either. Is it that length is unsigned
and the C compiler is being dumb about overflow?

Russ

[xi, 2013-03-05 08:07:05]

On 2013/01/31 15:50:59, rsc wrote:
> I don't understand what's wrong here either. Is it that length is unsigned
> and the C compiler is being dumb about overflow?

The compiler basically turns `p + length < p' into `length < 0', assuming
no-pointer-overflow.  I included a simplified example at:

https://codereview.appspot.com/7231069/

[rsc, 2013-03-19 18:35:20]

*** Submitted as
https://code.google.com/p/plan9port/source/detail?r=3cd53e993790 ***

libsec: avoid undefined C

gcc compiles `p + length < p' into 'length < 0' since pointer overflow is
undefined behavior in C.  This breaks the check against a large `length'.

Use `length > pend - p' instead.

There's no need to check `length < 0' since `length' is from length_decode() and
should be non-negative.

===

Try the simplified code.

void bar(void);
void foo(unsigned char *p, int length)
{
        if (p + length < p)
                bar();
}

$ gcc -S -o - t.c -O2
...
foo:
.LFB0:
        .cfi_startproc
        testl	%esi, %esi
        js	.L4
        rep
        ret
.L4:
        jmp	bar
        .cfi_endproc

Clearly `p' is not used at all.

R=rsc
CC=plan9port.codebot
https://codereview.appspot.com/7231069

Committer: Russ Cox <rsc@swtch.com>

[rsc, 2013-03-19 18:35:29]

thanks. sorry for the long delay. drowning in mail.