linux-hardening - Re: [RFC] Mitigating unexpected arithmetic overflow

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Date: Thu, 9 May 2024 10:08:54 -0400
From: "Theodore Ts'o" <tytso@....edu>
To: Kees Cook <keescook@...omium.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
        Justin Stitt <justinstitt@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mark Rutland <mark.rutland@....com>, linux-hardening@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [RFC] Mitigating unexpected arithmetic overflow

On Wed, May 08, 2024 at 11:11:35PM -0700, Kees Cook wrote:
> > I think it would be interesting in general to have some kind of
> > warning for "implicit cast drops bits".
> > 
> > I fear that we'd have an enormous about of them, and maybe they'd be
> > unsolvable without making the code *much* uglier (and sometimes the
> > fix might be to add an explicit cast to document intentionally dropped
> > bits, but explicit casts have their own issues).

Seapking of which, I recently had to work around an overactive
compiler UBSAN which complained about this:

struct ext2_super {
       ...
       __u32	time_lo;
       __u32	time_high;
       ...
}

	time_t	now;
	
	sb->time_low = now;
	sb->time_high = now >> 32;

This is obviously (to a human) correct, but because of stupid compiler
tricks, in order to silence compiler-level and ubsan complaints, this
got turned into:


	sb->time_low = now & 0xffffffff;
#if (SIZEOF_TIME_T > 4)
	sb->time_high = (now >> 32) & EXT4_EPOCH_MASK;
#else
	sb->time_high = 0;
#endif

and in the opposite case, I was forced to write:

#if (SIZEOF_TIME_T == 4)
	return *lo;
#else
	return ((time_t)(*hi) << 32) | *lo;
#endif

... and this made me very sad.  Grumble....

				- Ted