linux-kernel - Re: [PATCH v17 03/15] arm64: Introduce prctl() options to control the tagged user addresses ABI

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Date:   Mon, 17 Jun 2019 16:56:38 +0000
From:   Szabolcs Nagy <Szabolcs.Nagy@....com>
To:     Catalin Marinas <Catalin.Marinas@....com>,
        Andrey Konovalov <andreyknvl@...gle.com>
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Subject: Re: [PATCH v17 03/15] arm64: Introduce prctl() options to control the
 tagged user addresses ABI

On 17/06/2019 14:56, Catalin Marinas wrote:
> On Wed, Jun 12, 2019 at 01:43:20PM +0200, Andrey Konovalov wrote:
>> From: Catalin Marinas <catalin.marinas@....com>
>>
>> It is not desirable to relax the ABI to allow tagged user addresses into
>> the kernel indiscriminately. This patch introduces a prctl() interface
>> for enabling or disabling the tagged ABI with a global sysctl control
>> for preventing applications from enabling the relaxed ABI (meant for
>> testing user-space prctl() return error checking without reconfiguring
>> the kernel). The ABI properties are inherited by threads of the same
>> application and fork()'ed children but cleared on execve().
>>
>> The PR_SET_TAGGED_ADDR_CTRL will be expanded in the future to handle
>> MTE-specific settings like imprecise vs precise exceptions.
>>
>> Signed-off-by: Catalin Marinas <catalin.marinas@....com>
> 
> A question for the user-space folk: if an application opts in to this
> ABI, would you want the sigcontext.fault_address and/or siginfo.si_addr
> to contain the tag? We currently clear it early in the arm64 entry.S but
> we could find a way to pass it down if needed.

to me it makes sense to keep the tag in si_addr / fault_address.

but i don't know in detail how those fields are used currently.

keeping the tag is certainly useful for MTE to debug wrong tag
failures unless there is a separate mechanism for that.