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

[Ian Rogers]

On Thu, Oct 15, 2020 at 2:35 PM <hpa@xxxxxxxxx> wrote:
>
> 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.

On why this is undefined behavior:
https://lore.kernel.org/lkml/CAP-5=fU2XBoOa2=00VCuWYqsLUzMSMzUXY63ZJt9rz-NJ+vYwA@xxxxxxxxxxxxxx/

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

On performance, the memcpys are fixed size and so lowered to loads on
x86 by any reasonable compiler. See the thread above.

> 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.

The decoder is a shared code and using unaligned macros makes life
hard for the other users of the code. Memcpy is the "standard"
workaround for this kind of undefined behavior.
https://lore.kernel.org/lkml/e4269cb2-d8e6-da26-6afd-a9df72d4be36@xxxxxxxxx/

For motivation, beyond just having perf be sanitizer clean, see discussion here:
https://lore.kernel.org/lkml/CAP-5=fUoSGy3NAzTSbF3YLEPABSs7oPsxLkCx36XkEzzm341yw@xxxxxxxxxxxxxx/
https://lore.kernel.org/lkml/160208761921.7002.1321765913567405137.tip-bot2@tip-bot2/

Thanks,
Ian

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