Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [PATCH v10 3/4] random: introduce generic vDSO getrandom() implementation | Date | Wed, 30 Nov 2022 18:00:07 +0100 |
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On Wed, Nov 30 2022 at 16:47, Jason A. Donenfeld wrote: > On Wed, Nov 30, 2022 at 4:29 PM Arnd Bergmann <arnd@arndb.de> wrote: >> I see what you mean now. However this means your vdso32 copies >> are different between 32-bit and 64-bit kernels. If you need to >> access one of the fields from assembler, it even ends up >> different at source level, which adds a bit of complexity. >> >> Making the interface configuration-independent makes it obvious >> to the reader that none of these problems can happen. > > Except ideally, these are word-sized accesses (where only compat code > has to suffer I suppose).
While I hate it with a passion, there is actually a valid reason to use this ugly typedef.
On 32bit architectures which have load/store tearing of 64bit variables into two 32bit accesses due to ISA limitations, that results in undefined behaviour when write and read are concurrent. Neither READ_ONCE() nor WRITE_ONCE help there.
Though that begs the question whether we need a 64bit generation counter for the VDSO at all.
Thanks,
tglx
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