Peter Zijlstra says:

Address KCOV vs noinstr. There is no function attribute to selectively
 suppress KCOV instrumentation, instead teach objtool to NOP out the
 calls in noinstr functions.
 
 This cures a bunch of KCOV crashes (as used by syzcaller).
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Merge tag 'objtool_urgent_for_5.8_rc3' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull objtool fixes from Borislav Petkov:
 "Three fixes from Peter Zijlstra suppressing KCOV instrumentation in
  noinstr sections.

  Peter Zijlstra says:
    "Address KCOV vs noinstr. There is no function attribute to
     selectively suppress KCOV instrumentation, instead teach objtool
     to NOP out the calls in noinstr functions"

  This cures a bunch of KCOV crashes (as used by syzcaller)"

* tag 'objtool_urgent_for_5.8_rc3' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  objtool: Fix noinstr vs KCOV
  objtool: Provide elf_write_{insn,reloc}()
  objtool: Clean up elf_write() condition

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

@@ -0,0 +1,6 @@
+#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.
@@ -2766,7 +2785,7 @@ int check(const char *_objname, bool orc)
 
 	objname = _objname;
 
-	file.elf = elf_open_read(objname, orc ? O_RDWR : O_RDONLY);
+	file.elf = elf_open_read(objname, O_RDWR);
 	if (!file.elf)
 		return 1;
 
@@ -2827,7 +2846,9 @@ int check(const char *_objname, bool orc)
 		ret = create_orc_sections(&file);
 		if (ret < 0)
 			goto out;
+	}
 
+	if (file.elf->changed) {
 		ret = elf_write(file.elf);
 		if (ret < 0)
 			goto out;

tools/objtool/elf.c

@@ -529,8 +529,9 @@ static int read_relas(struct elf *elf)
 			rela->addend = rela->rela.r_addend;
 			rela->offset = rela->rela.r_offset;
 			symndx = GELF_R_SYM(rela->rela.r_info);
-			rela->sym = find_symbol_by_index(elf, symndx);
 			rela->sec = sec;
+			rela->idx = i;
+			rela->sym = find_symbol_by_index(elf, symndx);
 			if (!rela->sym) {
 				WARN("can't find rela entry symbol %d for %s",
 				     symndx, sec->name);
@@ -713,6 +714,8 @@ struct section *elf_create_section(struct elf *elf, const char *name,
 	elf_hash_add(elf->section_hash, &sec->hash, sec->idx);
 	elf_hash_add(elf->section_name_hash, &sec->name_hash, str_hash(sec->name));
 
+	elf->changed = true;
+
 	return sec;
 }
 
@@ -746,7 +749,7 @@ struct section *elf_create_rela_section(struct elf *elf, struct section *base)
 	return sec;
 }
 
-int elf_rebuild_rela_section(struct section *sec)
+int elf_rebuild_rela_section(struct elf *elf, struct section *sec)
 {
 	struct rela *rela;
 	int nr, idx = 0, size;
@@ -763,6 +766,9 @@ int elf_rebuild_rela_section(struct section *sec)
 		return -1;
 	}
 
+	sec->changed = true;
+	elf->changed = true;
+
 	sec->data->d_buf = relas;
 	sec->data->d_size = size;
 
@@ -779,7 +785,44 @@ int elf_rebuild_rela_section(struct section *sec)
 	return 0;
 }
 
-int elf_write(const struct elf *elf)
+int elf_write_insn(struct elf *elf, struct section *sec,
+		   unsigned long offset, unsigned int len,
+		   const char *insn)
+{
+	Elf_Data *data = sec->data;
+
+	if (data->d_type != ELF_T_BYTE || data->d_off) {
+		WARN("write to unexpected data for section: %s", sec->name);
+		return -1;
+	}
+
+	memcpy(data->d_buf + offset, insn, len);
+	elf_flagdata(data, ELF_C_SET, ELF_F_DIRTY);
+
+	elf->changed = true;
+
+	return 0;
+}
+
+int elf_write_rela(struct elf *elf, struct rela *rela)
+{
+	struct section *sec = rela->sec;
+
+	rela->rela.r_info = GELF_R_INFO(rela->sym->idx, rela->type);
+	rela->rela.r_addend = rela->addend;
+	rela->rela.r_offset = rela->offset;
+
+	if (!gelf_update_rela(sec->data, rela->idx, &rela->rela)) {
+		WARN_ELF("gelf_update_rela");
+		return -1;
+	}
+
+	elf->changed = true;
+
+	return 0;
+}
+
+int elf_write(struct elf *elf)
 {
 	struct section *sec;
 	Elf_Scn *s;
@@ -796,6 +839,8 @@ int elf_write(const struct elf *elf)
 				WARN_ELF("gelf_update_shdr");
 				return -1;
 			}
+
+			sec->changed = false;
 		}
 	}
 
@@ -808,6 +853,8 @@ int elf_write(const struct elf *elf)
 		return -1;
 	}
 
+	elf->changed = false;
+
 	return 0;
 }
 

tools/objtool/elf.h

@@ -64,9 +64,10 @@ struct rela {
 	GElf_Rela rela;
 	struct section *sec;
 	struct symbol *sym;
-	unsigned int type;
 	unsigned long offset;
+	unsigned int type;
 	int addend;
+	int idx;
 	bool jump_table_start;
 };
 
@@ -76,6 +77,7 @@ struct elf {
 	Elf *elf;
 	GElf_Ehdr ehdr;
 	int fd;
+	bool changed;
 	char *name;
 	struct list_head sections;
 	DECLARE_HASHTABLE(symbol_hash, ELF_HASH_BITS);
@@ -118,7 +120,11 @@ struct elf *elf_open_read(const char *name, int flags);
 struct section *elf_create_section(struct elf *elf, const char *name, size_t entsize, int nr);
 struct section *elf_create_rela_section(struct elf *elf, struct section *base);
 void elf_add_rela(struct elf *elf, struct rela *rela);
-int elf_write(const struct elf *elf);
+int elf_write_insn(struct elf *elf, struct section *sec,
+		   unsigned long offset, unsigned int len,
+		   const char *insn);
+int elf_write_rela(struct elf *elf, struct rela *rela);
+int elf_write(struct elf *elf);
 void elf_close(struct elf *elf);
 
 struct section *find_section_by_name(const struct elf *elf, const char *name);
@@ -130,7 +136,7 @@ struct rela *find_rela_by_dest(const struct elf *elf, struct section *sec, unsig
 struct rela *find_rela_by_dest_range(const struct elf *elf, struct section *sec,
 				     unsigned long offset, unsigned int len);
 struct symbol *find_func_containing(struct section *sec, unsigned long offset);
-int elf_rebuild_rela_section(struct section *sec);
+int elf_rebuild_rela_section(struct elf *elf, struct section *sec);
 
 #define for_each_sec(file, sec)						\
 	list_for_each_entry(sec, &file->elf->sections, list)

tools/objtool/orc_gen.c

@@ -222,7 +222,7 @@ int create_orc_sections(struct objtool_file *file)
 		}
 	}
 
-	if (elf_rebuild_rela_section(ip_relasec))
+	if (elf_rebuild_rela_section(file->elf, ip_relasec))
 		return -1;
 
 	return 0;