Date: Sun, 2 Aug 2020 19:35:23 -0600 From: Ariadne Conill <ariadne@...eferenced.org> To: musl@...ts.openwall.com, Rich Felker <dalias@...c.org> Subject: Re: [PATCH v4] implement recallocarray(3) Hello, On 2020-08-02 18:45, Rich Felker wrote: > On Sun, Aug 02, 2020 at 04:54:26PM -0600, Ariadne Conill wrote: >> This OpenBSD extension is similar to reallocarray(3), but >> zero-initializes the new memory area. >> >> This extension is placed in _BSD_SOURCE, like >> reallocarray(3). >> >> Changes from v3: >> - use calloc() instead of realloc() and always copy >> - explicitly zero old memory block >> - restore overflow checking for old size >> >> Changes from v2: >> - drop overflow checking for old size >> >> Changes from v1: >> - use realloc() instead of reallocarray() > > Can we finish discussion of questions raised before making new > versions with changes that haven't even been agreed upon? I asked > about EINVAL so we could determine if it's expected and if it's > reasonable to copy; that's not a demand to remove it. And the new > memset before free is a complete no-op. If there's a contract to > behave like explicit_bzero on the old memory, we may want to honor > that if we implement recallocarray, but that likely makes it entirely > unsuitable to the purpose you want it for and will bring back the > atrociously bad apk performance you previously saw with mallocng if > you use it there, since each increment of array size will incur a > whole memcpy of the existing array (quadratic time) rather than copies > only occuring a logarithmic number of times (yielding O(n log n) > overall) as would happen with realloc. At this time, there are no plans to use recallocarray(3) in apk, but that would only be a problem if used in a hot path. I don't envision any hot path in apk where we would ever use recallocarray(3). For managing buffers, we would continue using realloc() as we do now, since it's all bytes anyway. *My* use case for recallocarray(3) is simply that I want a reallocarray() that also zeros the new members, *and* I do not want to reimplement that function every time I write a program, because this is the sort of thing a libc should ideally provide in 2020. It provides calloc() after all, why not a realloc version of it? In order to do that, you have to have a similar interface to what recallocarray(3) uses, because you need to know where to start zeroing. There's not any way around it that is also interposable. I do agree that the hardness guarantees of the OpenBSD version implement significant overhead, and if we don't provide them, there will be some heartbleed type bug down the road. So, I stick with v4 patch behavior, because I do not wish for anyone to shoot themselves in the foot. Ariadne
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