linux-kernel - [PATCH v5 08/23] x86/kgdb: Avoid redundant comparison of patched code

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Message-ID: <20190426001143.4983-9-namit@vmware.com>
Date:   Thu, 25 Apr 2019 17:11:28 -0700
From:   Nadav Amit <namit@...are.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>
CC:     <linux-kernel@...r.kernel.org>, <x86@...nel.org>, <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nadav Amit <nadav.amit@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        <linux_dti@...oud.com>, <linux-integrity@...r.kernel.org>,
        <linux-security-module@...r.kernel.org>,
        <akpm@...ux-foundation.org>, <kernel-hardening@...ts.openwall.com>,
        <linux-mm@...ck.org>, <will.deacon@....com>,
        <ard.biesheuvel@...aro.org>, <kristen@...ux.intel.com>,
        <deneen.t.dock@...el.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Nadav Amit <namit@...are.com>
Subject: [PATCH v5 08/23] x86/kgdb: Avoid redundant comparison of patched code

text_poke() already ensures that the written value is the correct one
and fails if that is not the case. There is no need for an additional
comparison. Remove it.

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Nadav Amit <namit@...are.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
---
 arch/x86/kernel/kgdb.c | 14 +-------------
 1 file changed, 1 insertion(+), 13 deletions(-)

diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
index 2b203ee5b879..13b13311b792 100644
--- a/arch/x86/kernel/kgdb.c
+++ b/arch/x86/kernel/kgdb.c
@@ -747,7 +747,6 @@ void kgdb_arch_set_pc(struct pt_regs *regs, unsigned long ip)
 int kgdb_arch_set_breakpoint(struct kgdb_bkpt *bpt)
 {
 	int err;
-	char opc[BREAK_INSTR_SIZE];
 
 	bpt->type = BP_BREAKPOINT;
 	err = probe_kernel_read(bpt->saved_instr, (char *)bpt->bpt_addr,
@@ -766,11 +765,6 @@ int kgdb_arch_set_breakpoint(struct kgdb_bkpt *bpt)
 		return -EBUSY;
 	text_poke_kgdb((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr,
 		       BREAK_INSTR_SIZE);
-	err = probe_kernel_read(opc, (char *)bpt->bpt_addr, BREAK_INSTR_SIZE);
-	if (err)
-		return err;
-	if (memcmp(opc, arch_kgdb_ops.gdb_bpt_instr, BREAK_INSTR_SIZE))
-		return -EINVAL;
 	bpt->type = BP_POKE_BREAKPOINT;
 
 	return err;
@@ -778,9 +772,6 @@ int kgdb_arch_set_breakpoint(struct kgdb_bkpt *bpt)
 
 int kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt)
 {
-	int err;
-	char opc[BREAK_INSTR_SIZE];
-
 	if (bpt->type != BP_POKE_BREAKPOINT)
 		goto knl_write;
 	/*
@@ -791,10 +782,7 @@ int kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt)
 		goto knl_write;
 	text_poke_kgdb((void *)bpt->bpt_addr, bpt->saved_instr,
 		       BREAK_INSTR_SIZE);
-	err = probe_kernel_read(opc, (char *)bpt->bpt_addr, BREAK_INSTR_SIZE);
-	if (err || memcmp(opc, bpt->saved_instr, BREAK_INSTR_SIZE))
-		goto knl_write;
-	return err;
+	return 0;
 
 knl_write:
 	return probe_kernel_write((char *)bpt->bpt_addr,
-- 
2.17.1