Re: [PATCH v10 3/4] random: introduce generic vDSO getrandom() implementation

On Wed, Nov 30 2022 at 16:47, Jason A. Donenfeld wrote:

On Wed, Nov 30, 2022 at 4:29 PM Arnd Bergmann [off-list ref] wrote:

quoted

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