Re: [PATCH bpf-next 1/1] arm64: bpf: Add BPF exception tables

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

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On Thu, Jul 30, 2020 at 08:28:56AM -0400, Qian Cai wrote:
> On Tue, Jul 28, 2020 at 05:21:26PM +0200, Jean-Philippe Brucker wrote:
> > When a tracing BPF program attempts to read memory without using the
> > bpf_probe_read() helper, the verifier marks the load instruction with
> > the BPF_PROBE_MEM flag. Since the arm64 JIT does not currently recognize
> > this flag it falls back to the interpreter.
> > 
> > Add support for BPF_PROBE_MEM, by appending an exception table to the
> > BPF program. If the load instruction causes a data abort, the fixup
> > infrastructure finds the exception table and fixes up the fault, by
> > clearing the destination register and jumping over the faulting
> > instruction.
> > 
> > To keep the compact exception table entry format, inspect the pc in
> > fixup_exception(). A more generic solution would add a "handler" field
> > to the table entry, like on x86 and s390.
> > 
> > Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> 
> This will fail to compile on arm64,
> 
> https://gitlab.com/cailca/linux-mm/-/blob/master/arm64.config
> 
> arch/arm64/mm/extable.o: In function `fixup_exception':
> arch/arm64/mm/extable.c:19: undefined reference to `arm64_bpf_fixup_exception'

Thanks for the report, I attached a fix. Daniel, can I squash it and
resend as v2 or is it too late?

I'd be more confident if my patches sat a little longer on the list so
arm64 folks have a chance to review them. This isn't my first silly
mistake...

Thanks,
Jean

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From 17d0f041b57903cb2657dde15559cd1923498337 Mon Sep 17 00:00:00 2001
From: Jean-Philippe Brucker <jean-philippe@linaro.org>
Date: Thu, 30 Jul 2020 14:45:44 +0200
Subject: [PATCH] arm64: bpf: Fix build for !CONFIG_BPF_JIT

Add a stub for arm64_bpf_fixup_exception() when CONFIG_BPF_JIT isn't
enabled, and avoid the fixup in this case.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
---
 arch/arm64/include/asm/extable.h | 9 +++++++++
 arch/arm64/mm/extable.c          | 3 ++-
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/extable.h b/arch/arm64/include/asm/extable.h
index bcee40df1586..840a35ed92ec 100644
--- a/arch/arm64/include/asm/extable.h
+++ b/arch/arm64/include/asm/extable.h
@@ -22,8 +22,17 @@ struct exception_table_entry
 
 #define ARCH_HAS_RELATIVE_EXTABLE
 
+#ifdef CONFIG_BPF_JIT
 int arm64_bpf_fixup_exception(const struct exception_table_entry *ex,
 			      struct pt_regs *regs);
+#else /* !CONFIG_BPF_JIT */
+static inline
+int arm64_bpf_fixup_exception(const struct exception_table_entry *ex,
+			      struct pt_regs *regs)
+{
+	return 0;
+}
+#endif /* !CONFIG_BPF_JIT */
 
 extern int fixup_exception(struct pt_regs *regs);
 #endif
diff --git a/arch/arm64/mm/extable.c b/arch/arm64/mm/extable.c
index 1f42991cacdd..eee1732ab6cd 100644
--- a/arch/arm64/mm/extable.c
+++ b/arch/arm64/mm/extable.c
@@ -14,7 +14,8 @@ int fixup_exception(struct pt_regs *regs)
 	if (!fixup)
 		return 0;
 
-	if (regs->pc >= BPF_JIT_REGION_START &&
+	if (IS_ENABLED(CONFIG_BPF_JIT) &&
+	    regs->pc >= BPF_JIT_REGION_START &&
 	    regs->pc < BPF_JIT_REGION_END)
 		return arm64_bpf_fixup_exception(fixup, regs);
 
-- 
2.27.0