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Received: from 77-249-17-252.cable.dynamic.v4.ziggo.nl ([77.249.17.252] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1x9iwY-0000000Graq-2VlH; Thu, 24 Sep 2026 12:52:54 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 7EFB13002D9; Thu, 24 Sep 2026 14:52:53 +0200 (CEST) Date: Thu, 24 Sep 2026 14:52:53 +0200 From: Peter Zijlstra To: Mark Rutland Cc: Mukesh Kumar Chaurasiya , catalin.marinas@arm.com, will@kernel.org, maddy@linux.ibm.com, mpe@ellerman.id.au, npiggin@gmail.com, chleroy@kernel.org, ritesh.list@gmail.com, sshegde@linux.ibm.com, pjw@kernel.org, palmer@dabbelt.com, aou@eecs.berkeley.edu, alex@ghiti.fr, hca@linux.ibm.com, gor@linux.ibm.com, agordeev@linux.ibm.com, borntraeger@linux.ibm.com, svens@linux.ibm.com, tglx@kernel.org, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, ojeda@kernel.org, boqun@kernel.org, gary@garyguo.net, bjorn3_gh@protonmail.com, lossin@kernel.org, a.hindborg@kernel.org, aliceryhl@google.com, tmgross@umich.edu, dakr@kernel.org, daniel.almeida@collabora.com, tamird@kernel.org, acourbot@nvidia.com, work@onurozkan.dev, nathan@kernel.org, ndesaulniers@google.com, morbo@google.com, justinstitt@google.com, jszhang@kernel.org, japo@linux.ibm.com, jpoimboe@kernel.org, seanjc@google.com, pmladek@suse.com, thuth@redhat.com, ynorov@nvidia.com, joelagnelf@nvidia.com, david@davidgow.net, fujita.tomonori@gmail.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, rust-for-linux@vger.kernel.org, llvm@lists.linux.dev Subject: Re: [RFC] rust: kernel: Add KUnit tests for ARCH_WARN_ASM bug table emission Message-ID: <20260924125253.GY776954@noisy.programming.kicks-ass.net> References: <20260922054959.955093-2-mkchauras@gmail.com> <20260922064735.GS4121339@noisy.programming.kicks-ass.net> <20260924111557.GP1837346@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: llvm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Thu, Sep 24, 2026 at 01:46:59PM +0100, Mark Rutland wrote: > > diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c > > index 914282016069..3d8a969eab5f 100644 > > --- a/arch/arm64/kernel/traps.c > > +++ b/arch/arm64/kernel/traps.c > > @@ -987,14 +987,13 @@ void do_serror(struct pt_regs *regs, unsigned long esr) > > #ifdef CONFIG_GENERIC_BUG > > int is_valid_bugaddr(unsigned long addr) > > { > > - /* > > - * bug_brk_handler() only called for BRK #BUG_BRK_IMM. > > - * So the answer is trivial -- any spurious instances with no > > - * bug table entry will be rejected by report_bug() and passed > > - * back to the debug-monitors code and handled as a fatal > > - * unexpected debug exception. > > - */ > > - return 1; > > + u32 insn; > > + > > + if (aarch64_insn_read((u32 *)addr, &insn)) > > + return 0; > > + > > + /* Match ASM_BUG_FLAGS() / BUG() instructions. */ > > + return insn == (0xd4200000 | (BUG_BRK_IMM << 5)); > > } > > #endif > > What problem is this trying to solve? > > Mukesh's original patch seems to be trying to check whether we compiled > things correctly (such that the bug_table entry points at the expected > insturction), and I don't think that warrants a runtime check, but (as > per my reply to him) I don't understand why that's a thing to check in > the first place. > > On arm64 we only call is_valid_bugaddr() under do_el1_brk64() -> > call_el1_break_hook() -> bug_brk_handler(). We only call > bug_brk_handler() when HW has taken a BRK exception with the BUG_BRK_IMM > immediate, and that can only be triggered by executing the right > instruction (BRK #BUG_BRK_IMM), which we ONLY use for BUG(). and WARN(). > > So I don't see why any change is necessary here. > > If the HW has somehow taken a BRK exception from any other instruction > (which would presumably be a HW bug), there won't be a bug_table entry, > and so __report_bug() will fail to find an entry, and will return > BUG_TRAP_TYPE_NONE, same as for the !is_valid_bugaddr() case. Right, so I wasn't at all sure what he was doing, Rust is still mostly unreadable garbage to me. But I figured I'd much rather see him use is_valid_bugaddr() to do runtime checks than whatever dodgy games he was playing with that label. So *if* doing runtime checks is what is wanted, then is_valid_bugaddr() needs to actually work, because then it grows a user outside of the BRK handler. Lots of IFs etc, but since I am having a head-ache and am mostly useless, I figured I'd distract myself doing whatever :-)