-Wsometimes-uninitialized in arch/mips/kernel/branch.c

[nathanchance]

arch/mips/kernel/branch.c:148:8: warning: variable 'bc_false' is used uninitialized whenever switch case is taken [-Wsometimes-uninitialized]
                case mm_bc2t_op:
                     ^~~~~~~~~~
arch/mips/kernel/branch.c:157:8: note: uninitialized use occurs here
                        if (bc_false)
                            ^~~~~~~~
arch/mips/kernel/branch.c:149:8: warning: variable 'bc_false' is used uninitialized whenever switch case is taken [-Wsometimes-uninitialized]
                case mm_bc1t_op:
                     ^~~~~~~~~~
arch/mips/kernel/branch.c:157:8: note: uninitialized use occurs here
                        if (bc_false)
                            ^~~~~~~~
arch/mips/kernel/branch.c:142:4: note: variable 'bc_false' is declared here
                        int bc_false = 0;
                        ^
2 warnings generated.

Full code: https://github.com/torvalds/linux/blob/3cf5d076fb4d48979f382bc9452765bf8b79e740/arch/mips/kernel/branch.c#L139-L172

Tentative patch (some initial review would be appreciated before I send it along):

From b0192ebf114ad6e33c780de6a8f0ad7fcd2fd5c1 Mon Sep 17 00:00:00 2001
From: Nathan Chancellor <natechancellor@gmail.com>
Date: Wed, 17 Jul 2019 10:21:47 -0700
Subject: [PATCH] MIPS: Don't use bc_false uninitialized in __mm_isBranchInstr

clang warns:

arch/mips/kernel/branch.c:148:8: warning: variable 'bc_false' is used
uninitialized whenever switch case is taken [-Wsometimes-uninitialized]
                case mm_bc2t_op:
                     ^~~~~~~~~~
arch/mips/kernel/branch.c:157:8: note: uninitialized use occurs here
                        if (bc_false)
                            ^~~~~~~~
arch/mips/kernel/branch.c:149:8: warning: variable 'bc_false' is used
uninitialized whenever switch case is taken [-Wsometimes-uninitialized]
                case mm_bc1t_op:
                     ^~~~~~~~~~
arch/mips/kernel/branch.c:157:8: note: uninitialized use occurs here
                        if (bc_false)
                            ^~~~~~~~
arch/mips/kernel/branch.c:142:4: note: variable 'bc_false' is declared
here
                        int bc_false = 0;
                        ^
2 warnings generated.

When mm_bc1t_op and mm_bc2t_op are taken, the bc_false initialization
does not happen, which leads to a garbage value upon use, as illustrated
below with a small sample program.

$ mipsel-linux-gnu-gcc --version | head -n1
mipsel-linux-gnu-gcc (Debian 8.3.0-2) 8.3.0

$ clang --version | head -n1
ClangBuiltLinux clang version 9.0.0 (git://github.com/llvm/llvm-project
544315b4197034a3be8acd12cba56a75fb1f08dc) (based on LLVM 9.0.0svn)

$ cat test.c
 #include <stdio.h>

 static void switch_scoped(int opcode)
 {
	 switch (opcode) {
	 case 1:
	 case 2: {
		 int bc_false = 0;

		 bc_false = 4;
	 case 3:
	 case 4:
		 printf("\t* switch scoped bc_false = %d\n", bc_false);
	 }
	 }
 }

 static void function_scoped(int opcode)
 {
	 int bc_false = 0;

	 switch (opcode) {
	 case 1:
	 case 2: {
		 bc_false = 4;
	 case 3:
	 case 4:
		 printf("\t* function scoped bc_false = %d\n", bc_false);
	 }
	 }
 }

 int main(void)
 {
	 int opcode;

	 for (opcode = 1; opcode < 5; opcode++) {
		 printf("opcode = %d:\n", opcode);
		 switch_scoped(opcode);
		 function_scoped(opcode);
		 printf("\n");
	 }

	 return 0;
 }

$ mipsel-linux-gnu-gcc -std=gnu89 -static test.c && \
  qemu-mipsel a.out
opcode = 1:
        * switch scoped bc_false = 4
        * function scoped bc_false = 4

opcode = 2:
        * switch scoped bc_false = 4
        * function scoped bc_false = 4

opcode = 3:
        * switch scoped bc_false = 2147483004
        * function scoped bc_false = 0

opcode = 4:
        * switch scoped bc_false = 2147483004
        * function scoped bc_false = 0

$ clang -std=gnu89 --target=mipsel-linux-gnu -m32 -static test.c && \
  qemu-mipsel a.out
opcode = 1:
        * switch scoped bc_false = 4
        * function scoped bc_false = 4

opcode = 2:
        * switch scoped bc_false = 4
        * function scoped bc_false = 4

opcode = 3:
        * switch scoped bc_false = 2147483004
        * function scoped bc_false = 0

opcode = 4:
        * switch scoped bc_false = 2147483004
        * function scoped bc_false = 0

Move the definition up so that we get the right behavior and mark it
__maybe_unused as it will not be used when CONFIG_MIPS_FP_SUPPORT
isn't enabled.

Fixes: 6a1cc218b9cc ("MIPS: branch: Remove FP branch handling when CONFIG_MIPS_FP_SUPPORT=n")
Link: https://github.com/ClangBuiltLinux/linux/issues/603
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
---
 arch/mips/kernel/branch.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/mips/kernel/branch.c b/arch/mips/kernel/branch.c
index 1db29957a931..2c38f75d87ff 100644
--- a/arch/mips/kernel/branch.c
+++ b/arch/mips/kernel/branch.c
@@ -58,6 +58,7 @@ int __mm_isBranchInstr(struct pt_regs *regs, struct mm_decoded_insn dec_insn,
 		       unsigned long *contpc)
 {
 	union mips_instruction insn = (union mips_instruction)dec_insn.insn;
+	int __maybe_unused bc_false = 0;
 
 	if (!cpu_has_mmips)
 		return 0;
@@ -139,7 +140,6 @@ int __mm_isBranchInstr(struct pt_regs *regs, struct mm_decoded_insn dec_insn,
 #ifdef CONFIG_MIPS_FP_SUPPORT
 		case mm_bc2f_op:
 		case mm_bc1f_op: {
-			int bc_false = 0;
 			unsigned int fcr31;
 			unsigned int bit;
 
-- 
2.22.0

[nathanchance]

Patch sent: https://lore.kernel.org/lkml/20190812033120.43013-2-natechancellor@gmail.com/

[nathanchance]

Patch accepted: https://git.kernel.org/mips/c/c2869aafe7191d366d74c55cb8a93c6d0baba317

[nathanchance]

Merged into mainline: https://git.kernel.org/linus/c2869aafe7191d366d74c55cb8a93c6d0baba317