Re: [PATCH] futex: Fix fault_in_user_writeable()

[Thomas Gleixner]

Huacai,

On Tue, Aug 17 2021 at 15:38, Huacai Chen wrote:
> On Tue, Aug 17, 2021 at 3:07 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> On X86, it returns 0; on MIPS64 without patch, it hangs in kernel; on
> MIPS64 with this patch, it returns -1.

As expected.

> Then, I want to know, on "W implies R" archs (such as X86), should it
> return 0? Maybe return -1 is more reasonable? (because the VMA is
> marked as write-only). If this program should return -1, then I don't
> think this is a MIPS-specific problem.

No. mmap(.., PROT_WRITE...) is simply impossible on x86 and implies
PROT_READ as documented in mmap(2).

So why should this fail and only fail in the fault case, but succeed
when the PTE is already established?

Thanks,

        tglx