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Date:   Mon, 12 Feb 2018 16:19:35 +0100
From:   Oleg Nesterov <oleg@redhat.com>
To:     Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
Cc:     Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
        ananth@linux.vnet.ibm.com, lkml <linux-kernel@vger.kernel.org>
Subject: Re: Uprobe: Bug(?) when probing small binaries
Message-ID: <20180212151935.GA12577@redhat.com>
References: <e74318e0-0cdf-e55d-bf06-b481541e2697@linux.vnet.ibm.com>
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Hi Ravi,

On 02/12, Ravi Bangoria wrote:
>
> I'm observing a bug in the uprobe infrastructure.

I am not sure,

> When target binary
> is quite small, uprobe replaces 'trap' instruction at two different
> places. Ex,

but this is actually "the same place" (in the file), just 2 virtual addrs differ,

> ? (gdb) x/w 0x1001069c
> ??? 0x1001069c:??? 2080899750
>
> Now enable the probe:
>
> ? # echo 1 > events/probe_a/main/enable
>
> Check probed instruction:
>
> ? (gdb) disassemble main
> ?????? 0x000000001000069c <+8>:??? trap
>
> *Bug*:
>
> ? (gdb) x/w 0x1001069c
> ??? 0x1001069c:? 2145386504
>
> In short, when it replaces the probe instruction, it does some corruption
> in the readonly vma. This seems to be a bug.
>
> How did I get the other address 0x1001069c?I found build_map_info()
> returns these two vmas for the single probe:
>
> ? 10000000-10010000 r-xp 00000000 08:05 67325595?? /home/ravi/a.out
> ? 10010000-10020000 r--p 00000000 08:05 67325595?? /home/ravi/a.out

please note that these 2 vma's mmap the same region in a.out, so
*0x1001069c and *0x1000069c point to the same insn.

Oleg.