linux-kernel - Re: [PATCH v2] x86/insn, tools/x86: Fix some potential undefined behavior.

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Message-ID: <5E711F64-A9BF-48EC-83A3-3C644D80F848@zytor.com>
Date:   Thu, 15 Oct 2020 14:34:57 -0700
From:   hpa@...or.com
To:     Ian Rogers <irogers@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-kernel@...r.kernel.org,
        Adrian Hunter <adrian.hunter@...el.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
CC:     Numfor Mbiziwo-Tiapo <nums@...gle.com>
Subject: Re: [PATCH v2] x86/insn, tools/x86: Fix some potential undefined behavior.

On October 15, 2020 9:12:16 AM PDT, Ian Rogers <irogers@...gle.com> wrote:
>From: Numfor Mbiziwo-Tiapo <nums@...gle.com>
>
>Don't perform unaligned loads in __get_next and __peek_nbyte_next as
>these are forms of undefined behavior.
>
>These problems were identified using the undefined behavior sanitizer
>(ubsan) with the tools version of the code and perf test. Part of this
>patch was previously posted here:
>https://lore.kernel.org/lkml/20190724184512.162887-4-nums@google.com/
>
>v2. removes the validate_next check and merges the 2 changes into one
>as
>requested by Masami Hiramatsu <mhiramat@...nel.org>
>
>Signed-off-by: Ian Rogers <irogers@...gle.com>
>Signed-off-by: Numfor Mbiziwo-Tiapo <nums@...gle.com>
>---
> arch/x86/lib/insn.c       | 4 ++--
> tools/arch/x86/lib/insn.c | 4 ++--
> 2 files changed, 4 insertions(+), 4 deletions(-)
>
>diff --git a/arch/x86/lib/insn.c b/arch/x86/lib/insn.c
>index 404279563891..be88ab250146 100644
>--- a/arch/x86/lib/insn.c
>+++ b/arch/x86/lib/insn.c
>@@ -20,10 +20,10 @@
> 	((insn)->next_byte + sizeof(t) + n <= (insn)->end_kaddr)
> 
> #define __get_next(t, insn)	\
>-	({ t r = *(t*)insn->next_byte; insn->next_byte += sizeof(t); r; })
>+	({ t r; memcpy(&r, insn->next_byte, sizeof(t)); insn->next_byte +=
>sizeof(t); r; })
> 
> #define __peek_nbyte_next(t, insn, n)	\
>-	({ t r = *(t*)((insn)->next_byte + n); r; })
>+	({ t r; memcpy(&r, (insn)->next_byte + n, sizeof(t)); r; })
> 
> #define get_next(t, insn)	\
>	({ if (unlikely(!validate_next(t, insn, 0))) goto err_out;
>__get_next(t, insn); })
>diff --git a/tools/arch/x86/lib/insn.c b/tools/arch/x86/lib/insn.c
>index 0151dfc6da61..92358c71a59e 100644
>--- a/tools/arch/x86/lib/insn.c
>+++ b/tools/arch/x86/lib/insn.c
>@@ -20,10 +20,10 @@
> 	((insn)->next_byte + sizeof(t) + n <= (insn)->end_kaddr)
> 
> #define __get_next(t, insn)	\
>-	({ t r = *(t*)insn->next_byte; insn->next_byte += sizeof(t); r; })
>+	({ t r; memcpy(&r, insn->next_byte, sizeof(t)); insn->next_byte +=
>sizeof(t); r; })
> 
> #define __peek_nbyte_next(t, insn, n)	\
>-	({ t r = *(t*)((insn)->next_byte + n); r; })
>+	({ t r; memcpy(&r, (insn)->next_byte + n, sizeof(t)); r; })
> 
> #define get_next(t, insn)	\
>	({ if (unlikely(!validate_next(t, insn, 0))) goto err_out;
>__get_next(t, insn); })

Wait, what?

You are taking about x86-specific code, and on x86 unaligned memory accesses are supported, well-defined, and ubiquitous. 

This is B.S. at best, and unless the compiler turns the memcpy() right back into what you started with, deleterious for performance.

If you have a *very* good reason for this kind of churn, wrap it in the unaligned access macros, but using memcpy() is insane. All you are doing is making the code worse.

-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.