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Subject: Re: [RFC PATCH] x86/syscalls: allow tracing of __do_sys_[syscall]
 functions
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <20220913135213.720368-1-namit@vmware.com>
Date:   Mon, 19 Sep 2022 19:35:42 -0700
Cc:     LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>
Content-Transfer-Encoding: quoted-printable
Message-Id: <2D09BBFE-45F3-4B9C-8734-D002CA99FD94@gmail.com>
References: <20220913135213.720368-1-namit@vmware.com>
To:     "Steven Rostedt (Google)" <rostedt@goodmis.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>
Precedence: bulk

On Sep 13, 2022, at 6:52 AM, Nadav Amit <nadav.amit@gmail.com> wrote:

> From: Nadav Amit <namit@vmware.com>
>=20
> Tracing - through ftrace function tracer and kprobes - of certain =
common
> syscall functions is currently disabled. Setting kprobes on these
> functions is specifically useful for debugging of syscall failures.
>=20
> Such tracing is disabled since __do_sys_[syscall] functions are =
declared
> as "inline".  "inline" in the kernel is actually defined as a macro =
that
> in addition to using the inline keyword also disables tracing =
(notrace).
> According to the comments in the code, tracing inline functions can
> wreck havoc, which is probably true in some cases.
>=20
> In practice, however, this might be too extensive. The compiler =
regards
> the "inline" keyword only as a hint, which it is free to ignore. In
> fact, in my builds gcc ignores the "inline" hint for many
> __do_sys_[syscall] since some of these functions are quite big and
> called from multiple locations (for compat). As a result, these
> functions cannot be traced.
>=20
> There are 3 possible solutions for enabling the tracing of
> __do_sys_[syscall]:
>=20
> 1. Mark __do_sys_[syscall] as __always_inline instead of inline. This
>   would increase the executable size, which might not be desired.
>=20
> 2. Remove the inline hint from __do_sys_[syscall]. Again, it might
>   affect the generated code, inducing function call overhead for some
>   syscalls.
>=20
> 3. Remove "notrace" from the "inline" macro definition, and require
>   functions that cannot be traced to be marked explicitly as =
"notrace".
>   This might be the most correct solution, which would also enable =
tracing
>   of additional useful functions. But finding the functions that =
cannot
>   be traced is not easy without some automation.
>=20
> 4. Avoid the use of "notrace" specifically for __do_sys_[syscall].

Steven, Peter, assistance would be helpful now that you are hopefully =
less
busy.

Using kprobes, I repeatedly get various crashes such as the one below =
that
happen in functions that are called from do_idle(). I am not sure that I =
get
exactly the reason (although it seems to be related to the RCU). The =
traces
for some reason show that when I patch a 2 byte JCC instruction and the =
RIP
always points to the second byte of the instructions, which I would not
expect to ever happen.

So basically, I have two questions for you:

1. What is the reason that inline functions are marked with notrace?

2. Is probing function that is called from do_idle() supposed to work, =
or
   should the kernel prevent it?

Thanks,
Nadav


[ 2381.637652] BUG: unable to handle page fault for address: =
ffffc90077cb6e4b
[ 2381.645568] #PF: supervisor read access in kernel mode
[ 2381.651367] #PF: error_code(0x0000) - not-present page
[ 2381.657155] PGD 100000067 P4D 100000067 PUD 0=20
[ 2381.662154] Oops: 0000 [#1] PREEMPT SMP NOPTI
[ 2381.667040] CPU: 19 PID: 0 Comm: swapper/19 Not tainted 5.17.0-rc4 #6
[ 2381.674749] Hardware name: Cisco Systems Inc =
UCSC-C220-M5SX/UCSC-C220-M5SX, BIOS C220M5.4.0.1i.0.0522190226 =
05/22/2019
[ 2381.686733] RIP: 0010:poke_int3_handler+0x6d/0x140
[ 2381.692108] Code: 32 31 c0 48 81 c6 00 00 00 81 48 39 f1 74 39 f0 41 =
ff 08 c3 31 c0 c3 49 89 d3 49 89 c2 49 d1 ea 4c 89 d2 48 c1 e2 04 4c 01 =
da <48> 63 32 48 81 c6 00 00 00 81 48 39 f1 0f 82 b7 00 00 00 0f 87 98
[ 2381.713139] RSP: 0000:ffffc9000c6a7c90 EFLAGS: 00010086
[ 2381.718994] RAX: 000000000d4f7e0a RBX: 0000000000000000 RCX: =
ffffffff81226ffb
[ 2381.726990] RDX: ffffc90077cb6e4b RSI: ffffffff82400a99 RDI: =
ffffc9000c6a7cb8
[ 2381.734986] RBP: ffffc9000c6a7ca8 R08: ffffc9000d4f7d84 R09: =
ffffffff81226ffc
[ 2381.742979] R10: 0000000006a7bf05 R11: ffffc9000d4f7dfb R12: =
ffffc9000c6a7cb8
[ 2381.750974] R13: 0000000000000000 R14: 0000000000000000 R15: =
0000000000000000
[ 2381.758966] FS:  0000000000000000(0000) GS:ffff88afe3740000(0000) =
knlGS:0000000000000000
[ 2381.768031] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 2381.774469] CR2: ffffc90077cb6e4b CR3: 000000000680a001 CR4: =
00000000007706e0
[ 2381.782462] DR0: 0000000000000000 DR1: 0000000000000000 DR2: =
0000000000000000
[ 2381.790455] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: =
0000000000000400
[ 2381.798447] PKRU: 55555554
[ 2381.801481] Call Trace:
[ 2381.804224]  <TASK>
[ 2381.806575]  ? exc_int3+0x10/0xd0
[ 2381.810292]  asm_exc_int3+0x31/0x40
[ 2381.814205] RIP: 0010:__hrtimer_next_event_base+0x6c/0xe0
[ 2381.820260] Code: 01 00 00 00 48 8b 41 28 49 39 c6 74 67 48 c1 e6 06 =
48 8b 50 18 4a 2b 54 26 78 4c 39 ea 7d 08 49 89 d5 4d 85 f6 74 40 85 db =
cc <21> 44 89 f8 89 d9 f3 48 0f bc c9 d3 e0 89 ce 89 c9 48 83 c1 01 f7
[ 2381.841292] RSP: 0000:ffffc9000c6a7d98 EFLAGS: 00000046
[ 2381.847148] RAX: ffff88afe3763960 RBX: 0000000000000000 RCX: =
ffff88afe3763200
[ 2381.855141] RDX: 0000022a7fa0e539 RSI: 0000000000000000 RDI: =
ffff88afe37631c0
[ 2381.863133] RBP: ffffc9000c6a7dc8 R08: 00000066a1725854 R09: =
000002490d303b39
[ 2381.871117] R10: ffff88afe376fba4 R11: ffff88afe376fb84 R12: =
ffff88afe37631c0
[ 2381.879109] R13: 0000022a7fa0e539 R14: ffff88afe3763700 R15: =
0000000000000001
[ 2381.887107]  ? __hrtimer_next_event_base+0x6c/0xe0
[ 2381.892478]  elfcorehdr_read+0x40/0x40
[ 2381.896681]  tick_nohz_get_sleep_length+0x9d/0xc0
[ 2381.901955]  menu_select+0x4bb/0x630
[ 2381.905965]  cpuidle_select+0x16/0x20
[ 2381.910069]  do_idle+0x1d2/0x270
[ 2381.913689]  cpu_startup_entry+0x20/0x30
[ 2381.918086]  start_secondary+0x118/0x150
[ 2381.922484]  secondary_startup_64_no_verify+0xc3/0xcb
[ 2381.928147]  </TASK>
[ 2381.931535] Modules linked in: zram
[ 2381.936365] CR2: ffffc90077cb6e4b
[ 2381.940998] ---[ end trace 0000000000000000 ]---