linux-kernel - [PATCH] objtool: Improve UACCESS coverage

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Message-ID: <20190724164821.GB31425@hirez.programming.kicks-ass.net>
Date:   Wed, 24 Jul 2019 18:48:21 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        x86@...nel.org, Arnd Bergmann <arnd@...db.de>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH] objtool: Improve UACCESS coverage

On Wed, Jul 24, 2019 at 04:10:40PM +0200, Peter Zijlstra wrote:
> And that most certainly should trigger...
> 
> Let me gdb that objtool thing.

---
Subject: objtool: Improve UACCESS coverage

A clang build reported an (obvious) double CLAC while a GCC build did
not; it turns out we only re-visit instructions if the first visit was
with AC=0. If OTOH the first visit was with AC=1, we completely ignore
any subsequent visit, even when it has AC=0.

Fix this by using a visited mask, instead of boolean and (explicitly)
mark the AC state.

$ ./objtool check -b --no-fp --retpoline --uaccess ../../defconfig-build/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o
../../defconfig-build/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool: .altinstr_replacement+0x22: redundant UACCESS disable
../../defconfig-build/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool:   eb_copy_relocations.isra.34()+0xea: (alt)
../../defconfig-build/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool:   .altinstr_replacement+0xffffffffffffffff: (branch)
../../defconfig-build/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool:   eb_copy_relocations.isra.34()+0xd9: (alt)
../../defconfig-build/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool:   eb_copy_relocations.isra.34()+0xb2: (branch)
../../defconfig-build/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool:   eb_copy_relocations.isra.34()+0x39: (branch)
../../defconfig-build/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool:   eb_copy_relocations.isra.34()+0x0: <=== (func)

Reported-by: Josh Poimboeuf <jpoimboe@...hat.com>
Reported-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 tools/objtool/check.c | 7 ++++---
 tools/objtool/check.h | 3 ++-
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 5f26620f13f5..176f2f084060 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1946,6 +1946,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 	struct alternative *alt;
 	struct instruction *insn, *next_insn;
 	struct section *sec;
+	u8 visited;
 	int ret;
 
 	insn = first;
@@ -1972,12 +1973,12 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 			return 1;
 		}
 
+		visited = 1 << state.uaccess;
 		if (insn->visited) {
 			if (!insn->hint && !insn_state_match(insn, &state))
 				return 1;
 
-			/* If we were here with AC=0, but now have AC=1, go again */
-			if (insn->state.uaccess || !state.uaccess)
+			if (insn->visited & visited)
 				return 0;
 		}
 
@@ -2024,7 +2025,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 		} else
 			insn->state = state;
 
-		insn->visited = true;
+		insn->visited |= visited;
 
 		if (!insn->ignore_alts) {
 			bool skip_orig = false;
diff --git a/tools/objtool/check.h b/tools/objtool/check.h
index b881fafcf55d..6d875ca6fce0 100644
--- a/tools/objtool/check.h
+++ b/tools/objtool/check.h
@@ -33,8 +33,9 @@ struct instruction {
 	unsigned int len;
 	enum insn_type type;
 	unsigned long immediate;
-	bool alt_group, visited, dead_end, ignore, hint, save, restore, ignore_alts;
+	bool alt_group, dead_end, ignore, hint, save, restore, ignore_alts;
 	bool retpoline_safe;
+	u8 visited;
 	struct symbol *call_dest;
 	struct instruction *jump_dest;
 	struct instruction *first_jump_src;