From: Julien Thierry <jthierry@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>,
tglx@linutronix.de, linux-kernel@vger.kernel.org, x86@kernel.org,
mhiramat@kernel.org, mbenes@suse.cz,
Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH v2] objtool,ftrace: Implement UNWIND_HINT_RET_OFFSET
Date: Thu, 2 Apr 2020 07:41:46 +0100 [thread overview]
Message-ID: <684d6e29-4a01-b4a5-f906-7bdee5ad108f@redhat.com> (raw)
In-Reply-To: <20200401170910.GX20730@hirez.programming.kicks-ass.net>
On 4/1/20 6:09 PM, Peter Zijlstra wrote:
> On Wed, Apr 01, 2020 at 04:43:35PM +0100, Julien Thierry wrote:
>
>>> +static bool has_modified_stack_frame(struct instruction *insn, struct insn_state *state)
>>> {
>>> + u8 ret_offset = insn->ret_offset;
>>> int i;
>>>
>>> - if (state->cfa.base != initial_func_cfi.cfa.base ||
>>> - state->cfa.offset != initial_func_cfi.cfa.offset ||
>>> - state->stack_size != initial_func_cfi.cfa.offset ||
>>> - state->drap)
>>> + if (state->cfa.base != initial_func_cfi.cfa.base || state->drap)
>>> + return true;
>>> +
>>> + if (state->cfa.offset != initial_func_cfi.cfa.offset &&
>>> + !(ret_offset && state->cfa.offset == initial_func_cfi.cfa.offset + ret_offset))
>>
>> Isn't that the same thing as "state->cfa.offset !=
>> initial_func_cfi.cfa.offset + ret_offset" ?
>
> I'm confused on what cfa.offset is, sometimes it increase with
> stack_size, sometimes it doesn't.
>
Steven already replied for me about that :) .
> ISTR that for the ftrace case it was indeed cfa.offset + 8, but for the
> IRET case below (where it is now not used anymore) it was cfa.offset
> (not cfa.offset + 40, which I was expecting).
>
>>> + return true;
>>> +
>>> + if (state->stack_size != initial_func_cfi.cfa.offset + ret_offset)
>>> return true;
>>>
>>> - for (i = 0; i < CFI_NUM_REGS; i++)
>>> + for (i = 0; i < CFI_NUM_REGS; i++) {
>>> if (state->regs[i].base != initial_func_cfi.regs[i].base ||
>>> state->regs[i].offset != initial_func_cfi.regs[i].offset)
>>> return true;
>>> + }
>>>
>>> return false;
>>> }
>
>>> @@ -2185,6 +2148,13 @@ static int validate_branch(struct objtoo
>>>
>>> break;
>>>
>>> + case INSN_EXCEPTION_RETURN:
>>> + if (func) {
>>> + state.stack_size -= arch_exception_frame_size;
>>> + break;
>>
>> Why break instead of returning? Shouldn't an exception return mark the end
>> of a branch (whether inside or outside a function) ?
>>
>> Here it seems it will continue to the next instruction which might have been
>> unreachable.
>
> The code in question (x86's sync_core()), is an exception return to
> self. It pushes an exception frame that points to right after the
> exception return instruction.
>
> This is the only usage of IRET in STT_FUNC symbols.
>
> So rather than teaching objtool how to interpret the whole
> push;push;push;push;push;iret sequence, teach it how big the frame is
> (arch_exception_frame_size) and let it continue.
>
> All the other (real) IRETs are in STT_NOTYPE in the entry assembly.
>
Right, I see.. However I'm not completely convinced by this. I must
admit I haven't followed the whole conversation, but what was the issue
with the HINT_IRET_SELF? It seemed more elegant, but I might be missing
some context.
Otherwise, it might be worth having a comment in the code to point that
this only handles the sync_core() case.
Also, instead of adding a special "arch_exception_frame_size", I could
suggest:
- Picking this patch [1] from a completely arbitrary source
- Getting rid of INSN_STACK type, any instruction could then include
stack ops on top of their existing semantics, they can just have an
empty list if they don't touch SP/BP
- x86 decoder adds a stack_op to the iret to modify the stack pointer by
the right amount
[1] https://www.spinics.net/lists/kernel/msg3453725.html
Thanks,
--
Julien Thierry
next prev parent reply other threads:[~2020-04-02 6:41 UTC|newest]
Thread overview: 63+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-25 17:45 [PATCH v4 00/13] objtool: vmlinux.o and moinstr validation Peter Zijlstra
2020-03-25 17:45 ` [PATCH v4 01/13] objtool: Remove CFI save/restore special case Peter Zijlstra
2020-03-26 11:30 ` Peter Zijlstra
2020-03-26 12:58 ` Peter Zijlstra
2020-03-26 13:44 ` Josh Poimboeuf
2020-03-26 15:38 ` Peter Zijlstra
2020-03-27 4:19 ` Josh Poimboeuf
2020-03-26 14:44 ` Miroslav Benes
2020-03-26 15:04 ` Miroslav Benes
2020-03-26 13:00 ` Peter Zijlstra
2020-03-26 13:56 ` Josh Poimboeuf
2020-03-26 15:49 ` Peter Zijlstra
2020-03-26 19:57 ` Peter Zijlstra
2020-03-27 1:00 ` Josh Poimboeuf
2020-03-30 17:02 ` Peter Zijlstra
2020-03-30 19:02 ` Josh Poimboeuf
2020-03-30 20:02 ` Peter Zijlstra
2020-03-30 20:29 ` Peter Zijlstra
2020-03-31 11:16 ` [RFC][PATCH] objtool,ftrace: Implement UNWIND_HINT_RET_OFFSET Peter Zijlstra
2020-03-31 15:31 ` Steven Rostedt
2020-03-31 16:06 ` [RFC][PATCH] x86,ftrace: Shrink ftrace_regs_caller() by one byte Peter Zijlstra
2020-03-31 19:58 ` [RFC][PATCH] objtool,ftrace: Implement UNWIND_HINT_RET_OFFSET Peter Zijlstra
2020-03-31 20:26 ` Josh Poimboeuf
2020-03-31 20:23 ` Josh Poimboeuf
2020-03-31 20:40 ` Peter Zijlstra
2020-03-31 21:07 ` Peter Zijlstra
2020-03-31 21:17 ` Josh Poimboeuf
2020-03-31 21:20 ` Josh Poimboeuf
2020-03-31 22:27 ` [PATCH v2] " Peter Zijlstra
2020-04-01 14:14 ` Josh Poimboeuf
2020-04-01 14:22 ` Peter Zijlstra
2020-04-01 14:39 ` Josh Poimboeuf
2020-04-01 15:38 ` Peter Zijlstra
2020-04-01 15:39 ` Steven Rostedt
2020-04-01 15:43 ` Julien Thierry
2020-04-01 17:09 ` Peter Zijlstra
2020-04-01 17:33 ` Steven Rostedt
2020-04-01 17:45 ` Peter Zijlstra
2020-04-01 18:20 ` Steven Rostedt
2020-04-01 20:20 ` Peter Zijlstra
2020-04-01 17:37 ` Josh Poimboeuf
2020-04-02 6:41 ` Julien Thierry [this message]
2020-04-02 6:56 ` Julien Thierry
2020-04-02 7:50 ` Peter Zijlstra
2020-04-02 8:16 ` Julien Thierry
2020-04-02 8:17 ` Peter Zijlstra
2020-04-02 8:29 ` Julien Thierry
2020-04-02 8:58 ` Miroslav Benes
2020-03-25 17:45 ` [PATCH v4 02/13] objtool: Factor out CFI hints Peter Zijlstra
2020-03-25 18:26 ` Miroslav Benes
2020-03-25 19:41 ` Peter Zijlstra
2020-03-25 17:45 ` [PATCH v4 03/13] objtool: Rename struct cfi_state Peter Zijlstra
2020-03-25 17:45 ` [PATCH v4 04/13] objtool: Fix !CFI insn_state propagation Peter Zijlstra
2020-03-25 17:45 ` [PATCH v4 05/13] objtool: Implement noinstr validation Peter Zijlstra
2020-03-25 17:45 ` [PATCH v4 06/13] objtool: Optimize !vmlinux.o again Peter Zijlstra
2020-03-25 17:45 ` [PATCH v4 07/13] objtool: Use sec_offset_hash() for insn_hash Peter Zijlstra
2020-03-25 17:45 ` [PATCH v4 08/13] objtool: Detect loading function pointers across noinstr Peter Zijlstra
2020-03-25 17:45 ` [PATCH v4 09/13] kbuild/objtool: Add objtool-vmlinux.o pass Peter Zijlstra
2020-03-25 17:45 ` [PATCH v4 10/13] objtool: Avoid iterating !text section symbols Peter Zijlstra
2020-03-25 17:45 ` [PATCH v4 11/13] objtool: Rearrange validate_section() Peter Zijlstra
2020-03-25 17:45 ` [PATCH v4 12/13] objtool: Add STT_NOTYPE noinstr validation Peter Zijlstra
2020-03-25 17:45 ` [PATCH v4 13/13] objtool: Also consider .entry.text as noinstr Peter Zijlstra
2020-03-25 19:03 ` [PATCH v4 00/13] objtool: vmlinux.o and moinstr validation Miroslav Benes
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