Messages in this thread | | | Date | Mon, 17 Oct 2022 12:40:13 -0700 | From | Kees Cook <> | Subject | Re: [PATCH] pstore: migrate to crypto acomp interface (take 2) |
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On Mon, Oct 17, 2022 at 09:33:06PM +0200, Ard Biesheuvel wrote: > On Mon, 17 Oct 2022 at 21:29, Kees Cook <keescook@chromium.org> wrote: > > > > On Mon, Oct 17, 2022 at 08:14:14PM +0200, Ard Biesheuvel wrote: > > > So once we use the same size for input and output, I was curious > > > whether we could encrypt in place, and get rid of the big_oops_buf. > > > And the answer is 'yes', precisely because we have this horrid per-CPU > > > allocation which serves as a bounce buffer. And this is not specific > > > to acomp, the old comp algorithms get wrapped in scomps which receive > > > the same treatment. > > > > Ah, in the sense that "in place" is actually happening in the per-cpu > > allocation, and only if it succeeds does the input buffer get > > overwritten? > > Something like that IIRC. > > > > So at that point, I wondered what the point is of all this complexity. > > > Do we really need 6 different algorithms to compress a couple of K of > > > ASCII text on a code path that is ice cold by definition? Wouldn't it > > > be better to drop the crypto API altogether here, and just use GZIP > > > via the library interface? > > > > Well, my goal was to make the algo "pstore doesn't care". If someone > > picks deflate, do they still get all the per-cpu allocations? > > > > Not if you use the library interface directly. > > The issue with the percpu buffers is that they are only kept if any > scomp TFMs are active, but this is always the case for pstore, as you > don't want to allocate it on the panic path.
Okay, so strictly speaking, eliminating the per-CPU allocation is an improvement. Keeping scomp and doing in-place compression will let pstore use "any" compressions method.
Is there a crypto API that does _not_ preallocate the per-CPU stuff? Because, as you say, it's a huge amount of memory on the bigger systems...
-- Kees Cook
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