linux-kernel - Re: [RFC 1/3] abi_spec: basic definitions of constraints, args and syscalls

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Date:   Wed, 4 Jan 2017 04:52:54 +0000
From:   alexander.levin@...izon.com
To:     Dmitry Vyukov <dvyukov@...gle.com>
CC:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "scientist@...com" <scientist@...com>,
        "glider@...gle.com" <glider@...gle.com>,
        "andreyknvl@...gle.com" <andreyknvl@...gle.com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "arnd@...db.de" <arnd@...db.de>,
        "mathieu.desnoyers@...icios.com" <mathieu.desnoyers@...icios.com>,
        "daniel.vetter@...ll.ch" <daniel.vetter@...ll.ch>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 1/3] abi_spec: basic definitions of constraints, args and
 syscalls

On Wed, Dec 14, 2016 at 08:46:25PM +0100, Dmitry Vyukov wrote:
> Here is my current prototype:
> https://github.com/dvyukov/linux/commit/6200a9333e78bef393f8ead41205813b94d340f3
> 
> For now it can trace arguments of 4 system calls:
> 
> [    4.055483] [pid 1258] open(&00007ffdefc023a0=[], 0x0, 0x1b6)
> [    4.055991] [pid 1258] open(&00007ffdefc023a0=[], 0x0, 0x1b6) = 3
> [    4.056486] [pid 1258] read(0x3, &00007ffdefc01320=[], 0x1000)
> [    4.056977] [pid 1258] read(0x3, &00007ffdefc01320=[], 0x1000) = 1780
> [    4.057485] [pid 1258] read(0x3, &00007f316a732000=[], 0x1000)
> [    4.057991] [pid 1258] read(0x3, &00007f316a732000=[], 0x1000) = 0
> [    4.058488] [pid 1258] close(0x0) = 0
> [    4.058848] [pid 1258] write(0x1, &00007f316a732000=[], 0x5)
> [    4.059304] [pid 1258] write(0x1, &00007f316a732000=[], 0x5) = 5
> [    4.059905] [pid 1234] close(0x0) = 0
> [    4.060239] [pid 1234] close(0x0) = 0
> 
> 
> Main outstanding problems:
>  - understanding length of arrays and buffers
>  - understanding discriminators of unions and syscall variations

Happy new year! I've been away for a bit myself, but now back working on this.

Attached a patch on top of your commit.

There are two things (very messy, I just want to go through the concept):

 - Reading the values into a generic fields struct, based on your suggestion.
There's no actual struct there, just the values - we can figure out how it'll
look like exactly, but something along this path makes sense?

tglx also raised a point that we want to read from userspace only once for
performance; it's a bit early to address performance at this stage, but it's
another advantage to pursuing this approach.

 - Array/string length. Since we read all values, we can point to the array's
length by using an offset from the currect arg. So for example, in read(), the
length of the buffer is at +1 offset from the buffer. This seems to be the case
for most syscalls.

The exception here is strings which we can just define as offset == 0.


With the patch, the trace is now able to work with strings:

[    1.234156] [pid 891] open(&00007fa7b35d4035=[ /etc/ld.so.cache ], 0x80000, 0x1)
[    1.235244] [pid 891] open(&00007fa7b35d4035=[ /etc/ld.so.cache ], 0x80000, 0x1) = -2
[    1.236101] [pid 891] open(&00007ffe57ca2a70=[ /lib/x86_64-linux-gnu/tls/x86_64/libc.so.6 ], 0x80000, 0xb37db168)
[    1.237361] [pid 891] open(&00007ffe57ca2a70=[ /lib/x86_64-linux-gnu/tls/x86_64/libc.so.6 ], 0x80000, 0xb37db168) = -2
[    1.238545] [pid 891] open(&00007ffe57ca2a70=[ /lib/x86_64-linux-gnu/tls/libc.so.6 ], 0x80000, 0xb37db168)
[    1.239600] [pid 891] open(&00007ffe57ca2a70=[ /lib/x86_64-linux-gnu/tls/libc.so.6 ], 0x80000, 0xb37db168) = -2
[    1.241033] [pid 891] open(&00007ffe57ca2a70=[ /lib/x86_64-linux-gnu/x86_64/libc.so.6 ], 0x80000, 0xb37db168)
[    1.242163] [pid 891] open(&00007ffe57ca2a70=[ /lib/x86_64-linux-gnu/x86_64/libc.so.6 ], 0x80000, 0xb37db168) = -2
[    1.243329] [pid 891] open(&00007ffe57ca2a70=[ /lib/x86_64-linux-gnu/libc.so.6 ], 0x80000, 0xb37db168)
[    1.244712] [pid 891] open(&00007ffe57ca2a70=[ /lib/x86_64-linux-gnu/libc.so.6 ], 0x80000, 0xb37db168) = 3
[    1.245633] [pid 891] read(0x3, &00007ffe57ca2c98=[ (null) ], 0x340)
[    1.246334] [pid 891] read(0x3, &00007ffe57ca2c98=[ (null) ], 0x340) = 832

Does the idea makes sense?

-- 

Thanks,
Sasha

View attachment "diff.patch" of type "text/x-diff" (6635 bytes)