linux-kernel - Re: [tip:x86/microcode 1/1] arch/x86/kernel/cpu/microcode/amd.c:714:6: warning: variable 'equiv_id' is used uninitialized whenever 'if' condition is false

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Message-ID: <20240730080326.GAZqieTl1Gz4RheSiI@fat_crate.local>
Date: Tue, 30 Jul 2024 10:03:26 +0200
From: Borislav Petkov <bp@...en8.de>
To: Nathan Chancellor <nathan@...nel.org>
Cc: kernel test robot <lkp@...el.com>, llvm@...ts.linux.dev,
	oe-kbuild-all@...ts.linux.dev, linux-kernel@...r.kernel.org,
	x86@...nel.org
Subject: Re: [tip:x86/microcode 1/1]
 arch/x86/kernel/cpu/microcode/amd.c:714:6: warning: variable 'equiv_id' is
 used uninitialized whenever 'if' condition is false

On Mon, Jul 29, 2024 at 08:30:08AM -0700, Nathan Chancellor wrote:
> Right, as clang does not perform interprocedural analysis for the sake
> of warnings. That's partly why GCC's version of this warning was
> disabled in commit 78a5255ffb6a ("Stop the ad-hoc games with
> -Wno-maybe-initialized").

So why aren't we shutting up clang too?

> While it may be properly handled now, I think this pattern of
> conditionally avoiding using a variable uninitialized is dubious.

Well, in this particular case of the microcode loader, this is the best
compromise I can think of - I have two different paths of handling a microcode
patch - Zen and newer - and before - so I need those separate flows.

So sometimes it makes sense.

> So I guess you can just ignore this if you want but others (maybe even
> Linus) will likely notice this and report it as well.

I think this warning should be behind W=1 if it can trigger false positives.

Anyway:

From: "Borislav Petkov (AMD)" <bp@...en8.de>
Date: Tue, 30 Jul 2024 09:52:43 +0200
Subject: [PATCH] x86/microcode/AMD: Fix a -Wsometimes-uninitialized clang
 false positive

Initialize equiv_id in order to shut up:

  arch/x86/kernel/cpu/microcode/amd.c:714:6: warning: variable 'equiv_id' is \
  used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
        if (x86_family(bsp_cpuid_1_eax) < 0x17) {
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

because clang doesn't do interprocedural analysis for warnings to see
that this variable won't be used uninitialized.

Fixes: 94838d230a6c ("x86/microcode/AMD: Use the family,model,stepping encoded in the patch ID")
Reported-by: kernel test robot <lkp@...el.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202407291815.gJBST0P3-lkp@intel.com/
Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
---
 arch/x86/kernel/cpu/microcode/amd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/microcode/amd.c b/arch/x86/kernel/cpu/microcode/amd.c
index 1f5d36f92b8a..f63b051f25a0 100644
--- a/arch/x86/kernel/cpu/microcode/amd.c
+++ b/arch/x86/kernel/cpu/microcode/amd.c
@@ -703,7 +703,7 @@ static struct ucode_patch *find_patch(unsigned int cpu)
 {
 	struct ucode_cpu_info *uci = ucode_cpu_info + cpu;
 	u32 rev, dummy __always_unused;
-	u16 equiv_id;
+	u16 equiv_id = 0;
 
 	/* fetch rev if not populated yet: */
 	if (!uci->cpu_sig.rev) {
-- 
2.43.0


-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette