riscv: blacklist assembly symbols for kprobe

Adding kprobes on some assembly functions (mainly exception handling)
will result in crashes (either recursive trap or panic). To avoid such
errors, add ASM_NOKPROBE() macro which allow adding specific symbols
into the __kprobe_blacklist section and use to blacklist the following
symbols that showed to be problematic:
- handle_exception()
- ret_from_exception()
- handle_kernel_stack_overflow()

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20231004131009.409193-1-cleger@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/include/asm/asm.h

@@ -183,6 +183,16 @@
 	REG_L x31, PT_T6(sp)
 	.endm
 
+/* Annotate a function as being unsuitable for kprobes. */
+#ifdef CONFIG_KPROBES
+#define ASM_NOKPROBE(name)				\
+	.pushsection "_kprobe_blacklist", "aw";		\
+	RISCV_PTR name;					\
+	.popsection
+#else
+#define ASM_NOKPROBE(name)
+#endif
+
 #endif /* __ASSEMBLY__ */
 
 #endif /* _ASM_RISCV_ASM_H */

arch/riscv/kernel/entry.S

@@ -111,6 +111,7 @@ SYM_CODE_START(handle_exception)
 1:
 	tail do_trap_unknown
 SYM_CODE_END(handle_exception)
+ASM_NOKPROBE(handle_exception)
 
 /*
  * The ret_from_exception must be called with interrupt disabled. Here is the
@@ -184,6 +185,7 @@ SYM_CODE_START_NOALIGN(ret_from_exception)
 	sret
 #endif
 SYM_CODE_END(ret_from_exception)
+ASM_NOKPROBE(ret_from_exception)
 
 #ifdef CONFIG_VMAP_STACK
 SYM_CODE_START_LOCAL(handle_kernel_stack_overflow)
@@ -219,6 +221,7 @@ SYM_CODE_START_LOCAL(handle_kernel_stack_overflow)
 	move a0, sp
 	tail handle_bad_stack
 SYM_CODE_END(handle_kernel_stack_overflow)
+ASM_NOKPROBE(handle_kernel_stack_overflow)
 #endif
 
 SYM_CODE_START(ret_from_fork)