linux-kernel - [PATCH] drm/amd/display: Harden callers of division functions

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Message-ID: <20241231072806.4936-1-yangtiezhu@loongson.cn>
Date: Tue, 31 Dec 2024 15:28:06 +0800
From: Tiezhu Yang <yangtiezhu@...ngson.cn>
To: Alex Deucher <alexander.deucher@....com>,
	Christian König <christian.koenig@....com>,
	Xinhui Pan <Xinhui.Pan@....com>,
	Harry Wentland <harry.wentland@....com>,
	Leo Li <sunpeng.li@....com>,
	Rodrigo Siqueira <Rodrigo.Siqueira@....com>
Cc: Nathan Chancellor <nathan@...nel.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	loongarch@...ts.linux.dev,
	amd-gfx@...ts.freedesktop.org,
	llvm@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: [PATCH] drm/amd/display: Harden callers of division functions

There are objtool warnings compiled with the latest mainline LLVM:

  dc_fixpt_recip() falls through to next function dc_fixpt_sinc()
  spl_fixpt_recip() falls through to next function spl_fixpt_sinc()

Here are the call paths:

  dc_fixpt_recip()
    dc_fixpt_from_fraction()
      complete_integer_division_u64()
        div64_u64_rem()

  spl_fixpt_recip()
    spl_fixpt_from_fraction()
      spl_complete_integer_division_u64()
        spl_div64_u64_rem()

This was introduced by a change in Clang from a few months:

  [SimplifyCFG] Deduce paths unreachable if they cause div/rem UB)
  https://github.com/llvm/llvm-project/commit/37932643abab

Since the ASSERT does not do anything to prevent the divide by zero
(just flags it with WARN_ON) and the rest of the code doesn't either,
the callers of division functions should harden them against dividing
by zero to avoid undefined behavior.

Keep the current ASSERT for the aim of debugging, just add BUG() to
stop control flow if the divisior is zero.

Suggested-by: Nathan Chancellor <nathan@...nel.org>
Suggested-by: Xi Ruoyao <xry111@...111.site>
Suggested-by: Rui Wang <wangrui@...ngson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
Link: https://lore.kernel.org/lkml/20241220223403.GA2605890@ax162/
---
 drivers/gpu/drm/amd/display/dc/basics/fixpt31_32.c  | 1 +
 drivers/gpu/drm/amd/display/dc/spl/spl_fixpt31_32.c | 1 +
 2 files changed, 2 insertions(+)

diff --git a/drivers/gpu/drm/amd/display/dc/basics/fixpt31_32.c b/drivers/gpu/drm/amd/display/dc/basics/fixpt31_32.c
index 88d3f9d7dd55..e15391e36b40 100644
--- a/drivers/gpu/drm/amd/display/dc/basics/fixpt31_32.c
+++ b/drivers/gpu/drm/amd/display/dc/basics/fixpt31_32.c
@@ -52,6 +52,7 @@ static inline unsigned long long complete_integer_division_u64(
 	unsigned long long result;
 
 	ASSERT(divisor);
+	BUG_ON(!divisor);
 
 	result = div64_u64_rem(dividend, divisor, remainder);
 
diff --git a/drivers/gpu/drm/amd/display/dc/spl/spl_fixpt31_32.c b/drivers/gpu/drm/amd/display/dc/spl/spl_fixpt31_32.c
index 131f1e3949d3..ce2036950808 100644
--- a/drivers/gpu/drm/amd/display/dc/spl/spl_fixpt31_32.c
+++ b/drivers/gpu/drm/amd/display/dc/spl/spl_fixpt31_32.c
@@ -30,6 +30,7 @@ static inline unsigned long long spl_complete_integer_division_u64(
 	unsigned long long result;
 
 	SPL_ASSERT(divisor);
+	BUG_ON(!divisor);
 
 	result = spl_div64_u64_rem(dividend, divisor, remainder);
 
-- 
2.42.0