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

[hpa]

On October 15, 2020 9:12:16 AM PDT, Ian Rogers <irogers@xxxxxxxxxx> wrote:
>From: Numfor Mbiziwo-Tiapo <nums@xxxxxxxxxx>
>
>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@xxxxxxxxxx/
>
>v2. removes the validate_next check and merges the 2 changes into one
>as
>requested by Masami Hiramatsu <mhiramat@xxxxxxxxxx>
>
>Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
>Signed-off-by: Numfor Mbiziwo-Tiapo <nums@xxxxxxxxxx>
>---
> 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.

-- 
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