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

Thomas Gleixner
2022-11-30T17:00:10Z

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