linux-kernel - [PATCH v2 5/5] drm/amd/display: Harden callers of division functions

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Message-ID: <20250106090506.6334-1-yangtiezhu@loongson.cn>
Date: Mon,  6 Jan 2025 17:05:06 +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 v2 5/5] 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_WARN() and SPL_ASSERT_WARN() do not do anything to
prevent the divide by zero and the rest of the code does not either,
the callers of division functions should be hardened against dividing
by zero to avoid undefined behavior, so use the new added ASSERT_BUG()
and SPL_ASSERT_BUG() to stop the 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  | 2 +-
 drivers/gpu/drm/amd/display/dc/spl/spl_fixpt31_32.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

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 1e32a9ca7f49..de86eb049302 100644
--- a/drivers/gpu/drm/amd/display/dc/basics/fixpt31_32.c
+++ b/drivers/gpu/drm/amd/display/dc/basics/fixpt31_32.c
@@ -51,7 +51,7 @@ static inline unsigned long long complete_integer_division_u64(
 {
 	unsigned long long result;
 
-	ASSERT_WARN(divisor);
+	ASSERT_BUG(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 07a5c09b250e..d04a220fd2fe 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
@@ -29,7 +29,7 @@ static inline unsigned long long spl_complete_integer_division_u64(
 {
 	unsigned long long result;
 
-	SPL_ASSERT_WARN(divisor);
+	SPL_ASSERT_BUG(divisor);
 
 	result = spl_div64_u64_rem(dividend, divisor, remainder);
 
-- 
2.42.0