linux-kernel - Re: [PATCH v3] objtool: Fix SYSCALL instruction handling and INSN_CONTEXT

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Message-ID: <Z_K83hCo-IgEHvM_@gmail.com>
Date: Sun, 6 Apr 2025 19:41:50 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Juergen Gross <jgross@...e.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH v3] objtool: Fix SYSCALL instruction handling and
 INSN_CONTEXT_SWITCH


* Josh Poimboeuf <jpoimboe@...nel.org> wrote:

> The !CONFIG_IA32_EMULATION version of xen_entry_SYSCALL_compat() ends
> with a SYSCALL instruction.  In Xen, SYSCALL is a hypercall.  Usually
> that would return, but in this case it's a hypercall to IRET, which
> doesn't return.
> 
> Objtool doesn't know that, so it falls through to the next function,
> triggering a false positive:
> 
>   vmlinux.o: warning: objtool: xen_reschedule_interrupt+0x2a: RET before UNTRAIN
> 
> Fix that by adding UD2 after the SYSCALL to avoid the undefined behavior
> and prevent the objtool fallthrough, and teach validate_unret() to stop
> control flow on the UD2 like validate_branch() already does.
> 
> Unfortunately that's not the whole story.  While that works for
> validate_unret(), it breaks validate_branch() which terminates control
> flow after the SYSCALL, triggering an unreachable instruction warning on
> the UD2.
> 
> The real problem here is that INSN_CONTEXT_SWITCH is ambiguous.  It can
> represent both call semantics (SYSCALL, SYSENTER) and return semantics
> (SYSRET, IRET, RETS, RETU).  Those differ significantly: calls preserve
> control flow whereas returns terminate it.
> 
> validate_branch() uses an arbitrary rule for INSN_CONTEXT_SWITCH that
> almost works by accident: if in a function, keep going; otherwise stop.
> It should instead be based on the semantics of the underlying
> instruction.
> 
> INSN_CONTEXT_SWITCH's original purpose was to enable the "unsupported
> instruction in callable function" warning.  But that warning really has
> no reason to exist.  It has never found any bugs, and those instructions
> are only in entry code anyway.  So just get rid of it.
> 
> That in turn allows objtool to stop caring about SYSCALL or SYSENTER.
> Their call semantic means they usually don't affect control flow in the
> containing function/code, and can just be INSN_OTHER.  The far
> returns/jumps can also be ignored as those aren't used anywhere.
> 
> With SYSCALL and SYSENTER removed, INSN_CONTEXT_SWITCH now has a sane
> well-defined return semantic.  Rename it to INSN_EXCEPTION_RETURN to
> better reflect its meaning.

I think this should be split up into a series of ~3 patches. :-/

Thanks,

	Ingo