linux-kernel - Re: [PATCH] powerpc/perf: Dereference bhrb entries safely

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Date:   Tue, 12 Dec 2017 21:09:26 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     mpe@...erman.id.au,
        Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>
Cc:     benh@...nel.crashing.org, kamalesh@...ux.vnet.ibm.com,
        kan.liang@...el.com, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, maddy@...ux.vnet.ibm.com,
        paulus@...ba.org, tglx@...utronix.de
Subject: Re: [PATCH] powerpc/perf: Dereference bhrb entries safely

Ravi Bangoria wrote:
> It may very well happen that branch instructions recorded by
> bhrb entries already get unmapped before they get processed by
> the kernel. Hence, trying to dereference such memory location
> will endup in a crash. Ex,
> 
>     Unable to handle kernel paging request for data at address 0xc008000019c41764
>     Faulting instruction address: 0xc000000000084a14
>     NIP [c000000000084a14] branch_target+0x4/0x70
>     LR [c0000000000eb828] record_and_restart+0x568/0x5c0
>     Call Trace:
>     [c0000000000eb3b4] record_and_restart+0xf4/0x5c0 (unreliable)
>     [c0000000000ec378] perf_event_interrupt+0x298/0x460
>     [c000000000027964] performance_monitor_exception+0x54/0x70
>     [c000000000009ba4] performance_monitor_common+0x114/0x120
> 
> Fix this by deferefencing them safely.
> 
> Suggested-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
> Signed-off-by: Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>

Reviewed-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>


- Naveen