[PATCH -tip v2 1/3] x86/percpu: Fix "const_pcpu_hot" version generation failure

[PATCH -tip v2 1/3] x86/percpu: Fix "const_pcpu_hot" version generation failure

[Uros Bizjak]

Version generation for "const_pcpu_hot" symbol failed because genksyms
doesn't know the __seg_gs keyword. Revert commit 4604c052b84d
"x86/percpu: Declare const_pcpu_hot as extern const variable" and
use this_cpu_read_const instead to avoid "sparse: dereference of
noderef expression" warning when reading const_pcpu_hot.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
---
v2: Use BUILD_BUG() instead of BUG().
---
 arch/x86/include/asm/current.h   | 5 +++--
 arch/x86/include/asm/percpu.h    | 7 +++++++
 arch/x86/include/asm/processor.h | 2 +-
 3 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/current.h b/arch/x86/include/asm/current.h
index 9fbd7cb2dc86..c8c5674d69f6 100644
--- a/arch/x86/include/asm/current.h
+++ b/arch/x86/include/asm/current.h
@@ -37,12 +37,13 @@ static_assert(sizeof(struct pcpu_hot) == 64);
 DECLARE_PER_CPU_ALIGNED(struct pcpu_hot, pcpu_hot);
 
 /* const-qualified alias to pcpu_hot, aliased by linker. */
-extern const struct pcpu_hot __percpu_seg_override const_pcpu_hot;
+DECLARE_PER_CPU_ALIGNED(const struct pcpu_hot __percpu_seg_override,
+			const_pcpu_hot);
 
 static __always_inline struct task_struct *get_current(void)
 {
 	if (IS_ENABLED(CONFIG_USE_X86_SEG_SUPPORT))
-		return const_pcpu_hot.current_task;
+		return this_cpu_read_const(const_pcpu_hot.current_task);
 
 	return this_cpu_read_stable(pcpu_hot.current_task);
 }
diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index 0f12b2004b94..3859abad19ec 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -28,6 +28,7 @@
 
 #else /* ...!ASSEMBLY */
 
+#include <linux/build_bug.h>
 #include <linux/kernel.h>
 #include <linux/stringify.h>
 
@@ -462,6 +463,7 @@ do {									\
 #define this_cpu_write_8(pcp, val)	__raw_cpu_write(volatile, pcp, val)
 #endif
 
+#define this_cpu_read_const(pcp)	__raw_cpu_read(, pcp)
 #else /* CONFIG_USE_X86_SEG_SUPPORT */
 
 #define raw_cpu_read_1(pcp)		percpu_from_op(1, , "mov", pcp)
@@ -486,6 +488,11 @@ do {									\
 #define this_cpu_write_8(pcp, val)	percpu_to_op(8, volatile, "mov", (pcp), val)
 #endif
 
+/*
+ * The generic per-cpu infrastrucutre is not suitable for
+ * reading const-qualified variables.
+ */
+#define this_cpu_read_const(pcp)	({ BUILD_BUG(); (typeof(pcp))0; })
 #endif /* CONFIG_USE_X86_SEG_SUPPORT */
 
 #define raw_cpu_add_1(pcp, val)		percpu_add_op(1, , (pcp), val)
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 061aa86b4662..1188e8bf76a2 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -534,7 +534,7 @@ static __always_inline unsigned long current_top_of_stack(void)
 	 *  entry trampoline.
 	 */
 	if (IS_ENABLED(CONFIG_USE_X86_SEG_SUPPORT))
-		return const_pcpu_hot.top_of_stack;
+		return this_cpu_read_const(const_pcpu_hot.top_of_stack);
 
 	return this_cpu_read_stable(pcpu_hot.top_of_stack);
 }
-- 
2.42.0

[PATCH -tip v2 2/3] x86/traps: Use current_top_of_stack() helper in traps.c

[Uros Bizjak]

Use current_top_of_stack() helper in sync_regs() and vc_switch_off_ist()
instead of reading top_of_stack percpu variable explicitly.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
---
 arch/x86/kernel/traps.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index c876f1d36a81..78b1d1a6ed2c 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -772,7 +772,7 @@ DEFINE_IDTENTRY_RAW(exc_int3)
  */
 asmlinkage __visible noinstr struct pt_regs *sync_regs(struct pt_regs *eregs)
 {
-	struct pt_regs *regs = (struct pt_regs *)this_cpu_read(pcpu_hot.top_of_stack) - 1;
+	struct pt_regs *regs = (struct pt_regs *)current_top_of_stack() - 1;
 	if (regs != eregs)
 		*regs = *eregs;
 	return regs;
@@ -790,7 +790,7 @@ asmlinkage __visible noinstr struct pt_regs *vc_switch_off_ist(struct pt_regs *r
 	 * trust it and switch to the current kernel stack
 	 */
 	if (ip_within_syscall_gap(regs)) {
-		sp = this_cpu_read(pcpu_hot.top_of_stack);
+		sp = current_top_of_stack();
 		goto sync;
 	}
 
-- 
2.42.0

[PATCH -tip v2 3/3] x86/percpu: Avoid sparse warning with cast to named address space

[Uros Bizjak]

Currently sparse does not know about __seg_fs and __seg_fs named
address space qualifiers. Avoid thousands of warnings about unexpected
keyword at the end of cast operator by removing named address space
qualifier from __my_cpu_type() when __CHECKER__ is defined.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202310080853.UhMe5iWa-lkp@intel.com/
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
---
 arch/x86/include/asm/percpu.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index 3859abad19ec..cc4212d1491e 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -96,6 +96,15 @@
 
 #endif /* CONFIG_SMP */
 
+/*
+ * FIXME: Drop this hack once sparse learns how to ignore
+ * __seg_fs and __seg_gs named address space qualifiers.
+ */
+#ifdef __CHECKER__
+#undef __percpu_seg_override
+#define __percpu_seg_override
+#endif
+
 #define __my_cpu_type(var)	typeof(var) __percpu_seg_override
 #define __my_cpu_ptr(ptr)	(__my_cpu_type(*ptr) *)(uintptr_t)(ptr)
 #define __my_cpu_var(var)	(*__my_cpu_ptr(&var))
-- 
2.42.0

Re: [PATCH -tip v2 3/3] x86/percpu: Avoid sparse warning with cast to named address space

[Uros Bizjak]

On Mon, Dec 4, 2023 at 12:20 AM Uros Bizjak <ubizjak@gmail.com> wrote:
>
> Currently sparse does not know about __seg_fs and __seg_fs named
> address space qualifiers. Avoid thousands of warnings about unexpected
> keyword at the end of cast operator by removing named address space
> qualifier from __my_cpu_type() when __CHECKER__ is defined.

We have discussed this issue a bit on the linux-sparse ML [1], and the
proposed solution is to define:

+#define __seg_gs __attribute__((address_space(__seg_gs)))
+#define __seg_fs __attribute__((address_space(__seg_fs)))

as done in the attached patch.

[1] https://lore.kernel.org/linux-sparse/CAFULd4YMcHhkSR4P5tq3ghLQyUCkiTOKS6PQd1Bec-2NicUiwQ@mail.gmail.com/T/#md048c918cd2145c822727e3aa401326e1660842f

Uros.