Messages in this thread | | | Date | Thu, 15 Oct 2020 14:34:57 -0700 | Subject | Re: [PATCH v2] x86/insn, tools/x86: Fix some potential undefined behavior. | From | hpa@zytor ... |
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On October 15, 2020 9:12:16 AM PDT, Ian Rogers <irogers@google.com> wrote: >From: Numfor Mbiziwo-Tiapo <nums@google.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@kernel.org> > >Signed-off-by: Ian Rogers <irogers@google.com> >Signed-off-by: Numfor Mbiziwo-Tiapo <nums@google.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.
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