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Re: V2: [PATCH 01/24] x86: Rename __glibc_reserved1 to feature_1 in tcbhead_t [BZ #22563]

* Jeff Law:

> On 07/13/2018 01:05 PM, Carlos O'Donell wrote:
>> I think the sanitizers are not within their rights to use any bytes in the
>> tcbhead_t structure, particularly reserved bytes. We should coordinate with
>> them, but that should not stop the acceptance of this patch in 2.28.
> sanitizer_common/
> static bool Aarch64GetESR(ucontext_t *ucontext, u64 *esr) {
>   static const u32 kEsrMagic = 0x45535201;
>   u8 *aux = ucontext->uc_mcontext.__reserved;
>   while (true) {
>     _aarch64_ctx *ctx = (_aarch64_ctx *)aux;
>     if (ctx->size == 0) break;
>     if (ctx->magic == kEsrMagic) {
>       *esr = ((__sanitizer_esr_context *)ctx)->esr;
>       return true;
>     }
>     aux += ctx->size;
>   }
>   return false;
> }
> It's seriously lame that they're abusing the reserved field like that...

In this case, __reserved doesn't mean “reserved for future use”, but
“reserved as an allocation area for the kernel for dynamically sized
data structures”.  This is part of the kernel/userspace API.

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