linux-kernel - Re: Getting the way a SIGSEGV append when catching a SIGSEGV from within

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Message-ID: <2de78272-1cd7-ca1a-c3a3-950de331b727@nod.at>
Date:   Mon, 27 Mar 2017 21:20:47 +0200
From:   Richard Weinberger <richard@....at>
To:     none <ytrezq@...-eu.org>
Cc:     Linux Kernel <linux-kernel@...r.kernel.org>,
        linux-x86_64@...r.kernel.org
Subject: Re: Getting the way a SIGSEGV append when catching a SIGSEGV from
 within

Am 27.03.2017 um 20:55 schrieb none:
> Le 2017-03-27 17:30, Richard Weinberger a écrit :
>> On Mon, Mar 27, 2017 at 4:45 PM, none <ytrezq@...-eu.org> wrote:
>>> Hello,
>>>
>>> There’s three way to perform an invalid memory access :
>>>
>>> The attempt to execute/jump at an invalid address.
>>> The attempt to read at an invalid address.
>>> The attempt to write at an invalid address.
>>>
>>> Determining the execute case with rt_sigaction is easy : the last value of
>>> eip match the value of the address which caused the segfault.
>>>
>>> But how to know if the SIGSEGV occurred by a read or by a write attempt ? In
>>> the same time shouldn’t that information belong in the mmu ?
>>
>> Did you look at the machine specific context of SIGSEGV?
>> It will give you access to the error code and the trap number.
> 
> Sorry but so, in the case of x86_64, which is the struct member I need to look at ?

ucontext_t *c = context; // context is the 3rd parameter to your SIGSEGV handler when SA_SIGINFO is set
mcontext_t m = c->uc_mcontext;

m.gregs[REG_ERR] and m.gregs[REG_TRAPNO] are what you want.

HTH,
//richard