Messages in this thread | | | Date | Thu, 9 May 2024 19:48:06 +0100 | From | Al Viro <> | Subject | Re: [RFC] Mitigating unexpected arithmetic overflow |
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On Thu, May 09, 2024 at 11:39:04AM -0700, Linus Torvalds wrote:
> .. it might also actually be a good idea *IF* we were to have some > kind of "implicit cast drops bits" warning, in that the compiler for > that case wouldn't remove the upper bits calculation, but would > trigger a warning if they are non-zero. > > So there are actually potential advantages to just always apparently > doing the full 64-bit arithmetic. > > Without debug warnings, it's a no-op that the compiler will just skip. > And with some hypothetical debug flag, it would be a "you are now > losing the high bits of the time value when assigning the result to a > limited 32-bit time_t" warning.
FWIW, the thing that somewhat worries me about having a helper along the lines of combine_to_u64(low, high) is that foo->splat = combine_to_u64(something, something_else); would be inviting hard-to-catch brainos - foo->splat = combine_to_u64(something_else, something); would be very hard to catch on RTFS, especially when you'd been staring at that code for a long time. Explicitly spelled out it would be obvious which goes into bits 0..31 and which in 32..64.
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