linux-kernel - Re: [PATCH 5/9] x86/dumpstack: Improve opcodes dumping in the Code: section

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Date:   Fri, 9 Mar 2018 11:15:52 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, X86 ML <x86@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/9] x86/dumpstack: Improve opcodes dumping in the Code:
 section

On Thu, Mar 08, 2018 at 03:20:58PM -0800, Linus Torvalds wrote:
> Yes, so the reason the prologue is more important is that there's
> really two cases for the "Code:" line:

Yap, I had a hunch it must be about some of those but thanks for taking
the time and writing it down!

I've tried to summarize the reasoning and slap it as a comment above it
so that it is clear why we do it this way:

---
diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index c3aa0e29513e..7a21098c9128 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -71,6 +71,25 @@ static void printk_stack_address(unsigned long address, int reliable,
 	printk("%s %s%pB\n", log_lvl, reliable ? "" : "? ", (void *)address);
 }
 
+/*
+ * The are a couple of reasons for the 2/3rd prologue, courtesy of Linus:
+ *
+ * In case where we don't have the exact kernel image (which, if we did, we can
+ * simply disassemble and navigate to the RIP), the purpose of the bigger
+ * prologue is to have more context and to be able to correlate the code from
+ * the different toolchains better.
+ *
+ * In addition, it helps in recreating the register allocation of the failing
+ * kernel and thus make sense of the register dump.
+ *
+ * What is more, the additional complication of a variable length insn arch like
+ * x86 warrants having longer byte sequence before rIP so that the disassembler
+ * can "sync" up properly and find instruction boundaries when decoding the
+ * opcode bytes.
+ *
+ * Thus, the 2/3rds prologue and 64 byte OPCODE_BUFSIZE is just a random
+ * guesstimate in attempt to achieve all of the above.
+ */
 void show_opcodes(u8 *rip, const char *loglvl)
 {
 #define OPCODE_BUFSIZE 64

-- 
Regards/Gruss,
    Boris.

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