RISC-V Fixes for 6.3-rc2

* RISC-V architecture-specific ELF attributes have been disabled in the
   kernel builds.
 * A fix for a locking failure while during errata patching that
   manifests on SiFive-based systems.
 * A fix for a KASAN failure during stack unwinding.
 * A fix for some lockdep failures during text patching.
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Merge tag 'riscv-for-linus-6.3-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/riscv/linux

Pull RISC-V fixes from Palmer Dabbelt:

 - RISC-V architecture-specific ELF attributes have been disabled in the
   kernel builds

 - A fix for a locking failure while during errata patching that
   manifests on SiFive-based systems

 - A fix for a KASAN failure during stack unwinding

 - A fix for some lockdep failures during text patching

* tag 'riscv-for-linus-6.3-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/riscv/linux:
  RISC-V: Don't check text_mutex during stop_machine
  riscv: Use READ_ONCE_NOCHECK in imprecise unwinding stack mode
  RISC-V: fix taking the text_mutex twice during sifive errata patching
  RISC-V: Stop emitting attributes

arch/riscv/Makefile

@@ -84,6 +84,13 @@ endif
 # Avoid generating .eh_frame sections.
 KBUILD_CFLAGS += -fno-asynchronous-unwind-tables -fno-unwind-tables
 
+# The RISC-V attributes frequently cause compatibility issues and provide no
+# information, so just turn them off.
+KBUILD_CFLAGS += $(call cc-option,-mno-riscv-attribute)
+KBUILD_AFLAGS += $(call cc-option,-mno-riscv-attribute)
+KBUILD_CFLAGS += $(call as-option,-Wa$(comma)-mno-arch-attr)
+KBUILD_AFLAGS += $(call as-option,-Wa$(comma)-mno-arch-attr)
+
 KBUILD_CFLAGS_MODULE += $(call cc-option,-mno-relax)
 KBUILD_AFLAGS_MODULE += $(call as-option,-Wa$(comma)-mno-relax)
 

arch/riscv/errata/sifive/errata.c

@@ -111,7 +111,7 @@ void __init_or_module sifive_errata_patch_func(struct alt_entry *begin,
 			mutex_lock(&text_mutex);
 			patch_text_nosync(ALT_OLD_PTR(alt), ALT_ALT_PTR(alt),
 					  alt->alt_len);
-			mutex_lock(&text_mutex);
+			mutex_unlock(&text_mutex);
 			cpu_apply_errata |= tmp;
 		}
 	}

arch/riscv/include/asm/ftrace.h

@@ -109,6 +109,6 @@ int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec);
 #define ftrace_init_nop ftrace_init_nop
 #endif
 
-#endif
+#endif /* CONFIG_DYNAMIC_FTRACE */
 
 #endif /* _ASM_RISCV_FTRACE_H */

arch/riscv/include/asm/patch.h

@@ -9,4 +9,6 @@
 int patch_text_nosync(void *addr, const void *insns, size_t len);
 int patch_text(void *addr, u32 *insns, int ninsns);
 
+extern int riscv_patch_in_stop_machine;
+
 #endif /* _ASM_RISCV_PATCH_H */

arch/riscv/kernel/compat_vdso/Makefile

@@ -14,6 +14,10 @@ COMPAT_LD := $(LD)
 COMPAT_CC_FLAGS := -march=rv32g -mabi=ilp32
 COMPAT_LD_FLAGS := -melf32lriscv
 
+# Disable attributes, as they're useless and break the build.
+COMPAT_CC_FLAGS += $(call cc-option,-mno-riscv-attribute)
+COMPAT_CC_FLAGS += $(call as-option,-Wa$(comma)-mno-arch-attr)
+
 # Files to link into the compat_vdso
 obj-compat_vdso = $(patsubst %, %.o, $(compat_vdso-syms)) note.o
 

arch/riscv/kernel/ftrace.c

@@ -15,10 +15,19 @@
 void ftrace_arch_code_modify_prepare(void) __acquires(&text_mutex)
 {
 	mutex_lock(&text_mutex);
+
+	/*
+	 * The code sequences we use for ftrace can't be patched while the
+	 * kernel is running, so we need to use stop_machine() to modify them
+	 * for now.  This doesn't play nice with text_mutex, we use this flag
+	 * to elide the check.
+	 */
+	riscv_patch_in_stop_machine = true;
 }
 
 void ftrace_arch_code_modify_post_process(void) __releases(&text_mutex)
 {
+	riscv_patch_in_stop_machine = false;
 	mutex_unlock(&text_mutex);
 }
 
@@ -107,9 +116,9 @@ int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
 {
 	int out;
 
-	ftrace_arch_code_modify_prepare();
+	mutex_lock(&text_mutex);
 	out = ftrace_make_nop(mod, rec, MCOUNT_ADDR);
-	ftrace_arch_code_modify_post_process();
+	mutex_unlock(&text_mutex);
 
 	return out;
 }

arch/riscv/kernel/patch.c

@@ -11,6 +11,7 @@
 #include <asm/kprobes.h>
 #include <asm/cacheflush.h>
 #include <asm/fixmap.h>
+#include <asm/ftrace.h>
 #include <asm/patch.h>
 
 struct patch_insn {
@@ -20,6 +21,8 @@ struct patch_insn {
 	atomic_t cpu_count;
 };
 
+int riscv_patch_in_stop_machine = false;
+
 #ifdef CONFIG_MMU
 /*
  * The fix_to_virt(, idx) needs a const value (not a dynamic variable of
@@ -60,7 +63,14 @@ static int patch_insn_write(void *addr, const void *insn, size_t len)
 	 * Before reaching here, it was expected to lock the text_mutex
 	 * already, so we don't need to give another lock here and could
 	 * ensure that it was safe between each cores.
+	 *
+	 * We're currently using stop_machine() for ftrace & kprobes, and while
+	 * that ensures text_mutex is held before installing the mappings it
+	 * does not ensure text_mutex is held by the calling thread.  That's
+	 * safe but triggers a lockdep failure, so just elide it for that
+	 * specific case.
 	 */
+	if (!riscv_patch_in_stop_machine)
 		lockdep_assert_held(&text_mutex);
 
 	if (across_pages)
@@ -125,6 +135,7 @@ NOKPROBE_SYMBOL(patch_text_cb);
 
 int patch_text(void *addr, u32 *insns, int ninsns)
 {
+	int ret;
 	struct patch_insn patch = {
 		.addr = addr,
 		.insns = insns,
@@ -132,7 +143,18 @@ int patch_text(void *addr, u32 *insns, int ninsns)
 		.cpu_count = ATOMIC_INIT(0),
 	};
 
-	return stop_machine_cpuslocked(patch_text_cb,
-				       &patch, cpu_online_mask);
+	/*
+	 * kprobes takes text_mutex, before calling patch_text(), but as we call
+	 * calls stop_machine(), the lockdep assertion in patch_insn_write()
+	 * gets confused by the context in which the lock is taken.
+	 * Instead, ensure the lock is held before calling stop_machine(), and
+	 * set riscv_patch_in_stop_machine to skip the check in
+	 * patch_insn_write().
+	 */
+	lockdep_assert_held(&text_mutex);
+	riscv_patch_in_stop_machine = true;
+	ret = stop_machine_cpuslocked(patch_text_cb, &patch, cpu_online_mask);
+	riscv_patch_in_stop_machine = false;
+	return ret;
 }
 NOKPROBE_SYMBOL(patch_text);

arch/riscv/kernel/stacktrace.c

@@ -101,7 +101,7 @@ void notrace walk_stackframe(struct task_struct *task,
 	while (!kstack_end(ksp)) {
 		if (__kernel_text_address(pc) && unlikely(!fn(arg, pc)))
 			break;
-		pc = (*ksp++) - 0x4;
+		pc = READ_ONCE_NOCHECK(*ksp++) - 0x4;
 	}
 }