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AJvYcCUF2Q0EjSKd8qglMq+GdmYUjUJ5pe/I5c7D+kFyfvfzm36ujE9qz7eA5PWfYF1LYsOfG6Z+@lists.linux.dev X-Gm-Message-State: AOJu0Yw0R2S7qmgC7lgmu4YdFxGcXX0st58TxXplvFZVOSc4p2LVIylD rpmAsA4jWPAN4LnzJAgoCroGgpbjaId+mJLBBdvH7m6UEIwJV/TDZ8ymudI5cVhrFWVoXik1Y7m pfQ== X-Received: from wmol21.prod.google.com ([2002:a05:600c:47d5:b0:485:33e8:9296]) (user=elver job=prod-delivery.src-stubby-dispatcher) by 2002:a05:600c:b8a:b0:486:f308:94ec with SMTP id 5b1f17b1804b1-486f456fdeemr126886675e9.24.1773928377192; Thu, 19 Mar 2026 06:52:57 -0700 (PDT) Date: Thu, 19 Mar 2026 14:52:38 +0100 Precedence: bulk X-Mailing-List: llvm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 X-Mailer: git-send-email 2.53.0.851.ga537e3e6e9-goog Message-ID: <20260319135245.1420780-1-elver@google.com> Subject: [PATCH] compiler: Simplify generic RELOC_HIDE() From: Marco Elver To: elver@google.com, Peter Zijlstra Cc: Ingo Molnar , Nathan Chancellor , Nick Desaulniers , Bill Wendling , Justin Stitt , llvm@lists.linux.dev, Bart Van Assche , David Laight , linux-kernel@vger.kernel.org, Sean Christopherson Content-Type: text/plain; charset="UTF-8" When enabling Context Analysis (CONTEXT_ANALYSIS := y) in arch/x86/kvm code, Clang's Thread Safety Analysis failed to recognize that identical per_cpu() accesses refer to the same lock: | CC [M] arch/x86/kvm/vmx/posted_intr.o | arch/x86/kvm/vmx/posted_intr.c:186:2: error: releasing raw_spinlock '__ptr + __per_cpu_offset[vcpu->cpu]' that was not held [-Werror,-Wthread-safety-analysis] | 186 | raw_spin_unlock(&per_cpu(wakeup_vcpus_on_cpu_lock, vcpu->cpu)); | | ^ | ./include/linux/spinlock.h:276:32: note: expanded from macro 'raw_spin_unlock' | 276 | #define raw_spin_unlock(lock) _raw_spin_unlock(lock) | | ^ | arch/x86/kvm/vmx/posted_intr.c:207:1: error: raw_spinlock '__ptr + __per_cpu_offset[vcpu->cpu]' is still held at the end of function [-Werror,-Wthread-safety-analysis] | 207 | } | | ^ | arch/x86/kvm/vmx/posted_intr.c:182:2: note: raw_spinlock acquired here | 182 | raw_spin_lock_nested(&per_cpu(wakeup_vcpus_on_cpu_lock, vcpu->cpu), | | ^ | ./include/linux/spinlock.h:235:2: note: expanded from macro 'raw_spin_lock_nested' | 235 | _raw_spin_lock(((void)(subclass), (lock))) | | ^ | 2 errors generated. This occurred because the default RELOC_HIDE() implementation (used by the per-CPU macros) is a statement expression containing an intermediate 'unsigned long' variable (this version appears to predate Git history). While the analysis strips away inner casts when resolving pointer aliases, it stops when encountering intermediate non-pointer variables (this is Thread Safety Analysis specific and irrelevant for codegen). This prevents the analysis from concluding that the pointers passed to e.g. raw_spin_lock() and raw_spin_unlock() were identical when per-CPU accessors are used. Simplify RELOC_HIDE() to a single expression. This preserves the intent of obfuscating UB-introducing out-of-bounds pointer calculations from the compiler via the 'unsigned long' cast, but allows the alias analysis to successfully resolve the pointers. Using a recent Clang version, I observe that generated code remains the same for vmlinux; the intermediate variable was already being optimized away (for any respectable modern compiler, not doing so would be an optimizer bug). Note that GCC provides its own version of RELOC_HIDE(), so this change only affects Clang builds. Add a test case to lib/test_context-analysis.c to catch any regressions. Link: https://lore.kernel.org/all/e3946223-4543-4a76-a328-9c6865e95192@acm.org/ Reported-by: Bart Van Assche Reported-by: Sean Christopherson Signed-off-by: Marco Elver --- include/linux/compiler.h | 5 +---- lib/test_context-analysis.c | 11 +++++++++++ 2 files changed, 12 insertions(+), 4 deletions(-) diff --git a/include/linux/compiler.h b/include/linux/compiler.h index af16624b29fd..cb2f6050bdf7 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -149,10 +149,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val, #endif #ifndef RELOC_HIDE -# define RELOC_HIDE(ptr, off) \ - ({ unsigned long __ptr; \ - __ptr = (unsigned long) (ptr); \ - (typeof(ptr)) (__ptr + (off)); }) +# define RELOC_HIDE(ptr, off) ((typeof(ptr))((unsigned long)(ptr) + (off))) #endif #define absolute_pointer(val) RELOC_HIDE((void *)(val), 0) diff --git a/lib/test_context-analysis.c b/lib/test_context-analysis.c index 140efa8a9763..06b4a6a028e0 100644 --- a/lib/test_context-analysis.c +++ b/lib/test_context-analysis.c @@ -596,3 +596,14 @@ static void __used test_ww_mutex_lock_ctx(struct test_ww_mutex_data *d) ww_mutex_destroy(&d->mtx); } + +static DEFINE_PER_CPU(raw_spinlock_t, test_per_cpu_lock); + +static void __used test_per_cpu(int cpu) +{ + raw_spin_lock(&per_cpu(test_per_cpu_lock, cpu)); + raw_spin_unlock(&per_cpu(test_per_cpu_lock, cpu)); + + raw_spin_lock(per_cpu_ptr(&test_per_cpu_lock, cpu)); + raw_spin_unlock(per_cpu_ptr(&test_per_cpu_lock, cpu)); +} -- 2.53.0.851.ga537e3e6e9-goog