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Date:   Fri, 31 Jan 2020 15:27:27 +0100
From:   Borislav Petkov <bp@suse.de>
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     "Luck, Tony" <tony.luck@intel.com>, Ingo Molnar <mingo@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [GIT PULL] x86/asm changes for v5.6
Message-ID: <20200131142727.GB14851@zn.tnic>
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+ Josh.

On Thu, Jan 30, 2020 at 10:02:54AM -0800, Linus Torvalds wrote:
> Maybe with the exception that a short conditional jump inside the
> alternative code itself is fine.
> 
> Because a branch-over inside the alternative sequence (or a loop -
> think inline cmpxchg loop or whatever) would be fine, since it's
> unaffected by code placement.

Perhaps something like the below as a start.

The build becomes really noisy if the error case is hit because aborting
in handle_group_alt() really messes up objtool processing but that is
perhaps ok as the idea is to *see* that something's wrong.

---
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 4768d91c6d68..b4dfd625842d 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -756,7 +771,9 @@ static int handle_group_alt(struct objtool_file *file,
 	last_new_insn = NULL;
 	insn = *new_insn;
 	sec_for_each_insn_from(file, insn) {
-		if (insn->offset >= special_alt->new_off + special_alt->new_len)
+		unsigned long new_offset = special_alt->new_off + special_alt->new_len;
+
+		if (insn->offset >= new_offset)
 			break;
 
 		last_new_insn = insn;
@@ -765,8 +787,12 @@ static int handle_group_alt(struct objtool_file *file,
 		insn->func = orig_insn->func;
 
 		if (insn->type != INSN_JUMP_CONDITIONAL &&
-		    insn->type != INSN_JUMP_UNCONDITIONAL)
+		    insn->type != INSN_JUMP_UNCONDITIONAL) {
 			continue;
+		} else {
+			if (insn->offset > 0 && insn->offset + insn->len < new_offset)
+				ERROR("Subsequent JMP instructions are not alternatives-patched. Stopping.");
+		}
 
 		if (!insn->immediate)
 			continue;
diff --git a/tools/objtool/warn.h b/tools/objtool/warn.h
index cbb0a02b7480..652c7adc7650 100644
--- a/tools/objtool/warn.h
+++ b/tools/objtool/warn.h
@@ -40,6 +40,14 @@ static inline char *offstr(struct section *sec, unsigned long offset)
 	return str;
 }
 
+#define ERROR(format, ...)				\
+({							\
+	fprintf(stderr,					\
+		"%s: error: objtool: " format "\n",	\
+		objname, ##__VA_ARGS__);		\
+	abort();					\
+})
+
 #define WARN(format, ...)				\
 	fprintf(stderr,					\
 		"%s: warning: objtool: " format "\n",	\

-- 
Regards/Gruss,
    Boris.

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