objtool: Fix noinstr vs KCOV

Since many compilers cannot disable KCOV with a function attribute,
help it to NOP out any __sanitizer_cov_*() calls injected in noinstr
code.

This turns:

12:   e8 00 00 00 00          callq  17 <lockdep_hardirqs_on+0x17>
		13: R_X86_64_PLT32      __sanitizer_cov_trace_pc-0x4

into:

12:   0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
		13: R_X86_64_NONE      __sanitizer_cov_trace_pc-0x4

Just like recordmcount does.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Dmitry Vyukov <dvyukov@google.com>

arch/x86/Kconfig

@@ -67,7 +67,7 @@ config X86
 	select ARCH_HAS_FILTER_PGPROT
 	select ARCH_HAS_FORTIFY_SOURCE
 	select ARCH_HAS_GCOV_PROFILE_ALL
-	select ARCH_HAS_KCOV			if X86_64
+	select ARCH_HAS_KCOV			if X86_64 && STACK_VALIDATION
 	select ARCH_HAS_MEM_ENCRYPT
 	select ARCH_HAS_MEMBARRIER_SYNC_CORE
 	select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE

tools/objtool/arch.h

@@ -84,4 +84,6 @@ unsigned long arch_jump_destination(struct instruction *insn);
 
 unsigned long arch_dest_rela_offset(int addend);
 
+const char *arch_nop_insn(int len);
+
 #endif /* _ARCH_H */

tools/objtool/arch/x86/decode.c

@@ -565,3 +565,21 @@ void arch_initial_func_cfi_state(struct cfi_init_state *state)
 	state->regs[16].base = CFI_CFA;
 	state->regs[16].offset = -8;
 }
+
+const char *arch_nop_insn(int len)
+{
+	static const char nops[5][5] = {
+		/* 1 */ { 0x90 },
+		/* 2 */ { 0x66, 0x90 },
+		/* 3 */ { 0x0f, 0x1f, 0x00 },
+		/* 4 */ { 0x0f, 0x1f, 0x40, 0x00 },
+		/* 5 */ { 0x0f, 0x1f, 0x44, 0x00, 0x00 },
+	};
+
+	if (len < 1 || len > 5) {
+		WARN("invalid NOP size: %d\n", len);
+		return NULL;
+	}
+
+	return nops[len-1];
+}

tools/objtool/arch/x86/include/arch_elf.h

+#ifndef _OBJTOOL_ARCH_ELF
+#define _OBJTOOL_ARCH_ELF
+
+#define R_NONE R_X86_64_NONE
+
+#endif /* _OBJTOOL_ARCH_ELF */

tools/objtool/check.c

@@ -12,6 +12,7 @@
 #include "check.h"
 #include "special.h"
 #include "warn.h"
+#include "arch_elf.h"
 
 #include <linux/hashtable.h>
 #include <linux/kernel.h>
@@ -765,6 +766,24 @@ static int add_call_destinations(struct objtool_file *file)
 		} else
 			insn->call_dest = rela->sym;
 
+		/*
+		 * Many compilers cannot disable KCOV with a function attribute
+		 * so they need a little help, NOP out any KCOV calls from noinstr
+		 * text.
+		 */
+		if (insn->sec->noinstr &&
+		    !strncmp(insn->call_dest->name, "__sanitizer_cov_", 16)) {
+			if (rela) {
+				rela->type = R_NONE;
+				elf_write_rela(file->elf, rela);
+			}
+
+			elf_write_insn(file->elf, insn->sec,
+				       insn->offset, insn->len,
+				       arch_nop_insn(insn->len));
+			insn->type = INSN_NOP;
+		}
+
 		/*
 		 * Whatever stack impact regular CALLs have, should be undone
 		 * by the RETURN of the called function.