[RFC PATCH 9/9] arm64: ftrace: Simplify PLT handling

[Ard Biesheuvel]

From: Ard Biesheuvel <ardb@xxxxxxxxxx>

Instead of emitting the ftrace trampoline PLT into a separate ELF
section, allocate them at the start of the ordinary PLT array, so
that no special sections are needed.

Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
---
 arch/arm64/include/asm/module.lds.h |  1 -
 arch/arm64/kernel/module-plts.c     | 18 +++++-------------
 arch/arm64/kernel/module.c          |  9 +--------
 3 files changed, 6 insertions(+), 22 deletions(-)

diff --git a/arch/arm64/include/asm/module.lds.h b/arch/arm64/include/asm/module.lds.h
index 24fc9401c514..95540e956b41 100644
--- a/arch/arm64/include/asm/module.lds.h
+++ b/arch/arm64/include/asm/module.lds.h
@@ -1,6 +1,5 @@
 SECTIONS {
 	.plt 0 : { BYTE(0) }
-	.text.ftrace_trampoline 0 : { BYTE(0) }
 
 #ifdef CONFIG_KASAN_SW_TAGS
 	/*
diff --git a/arch/arm64/kernel/module-plts.c b/arch/arm64/kernel/module-plts.c
index e30876b90e30..6900d2242f1b 100644
--- a/arch/arm64/kernel/module-plts.c
+++ b/arch/arm64/kernel/module-plts.c
@@ -275,12 +275,14 @@ static int partition_branch_plt_relas(Elf64_Sym *syms, Elf64_Rela *rela,
 	return i;
 }
 
+static const int ftrace_plts = IS_ENABLED(CONFIG_DYNAMIC_FTRACE) ? NR_FTRACE_PLTS : 0;
+
 int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
 			      char *secstrings, struct module *mod)
 {
-	unsigned long core_plts = 0;
+	unsigned long core_plts = ftrace_plts;
 	Elf64_Sym *syms = NULL;
-	Elf_Shdr *pltsec, *tramp = NULL;
+	Elf_Shdr *pltsec;
 	int i;
 
 	/*
@@ -290,9 +292,6 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
 	for (i = 0; i < ehdr->e_shnum; i++) {
 		if (!strcmp(secstrings + sechdrs[i].sh_name, ".plt"))
 			mod->arch.core.plt_shndx = i;
-		else if (!strcmp(secstrings + sechdrs[i].sh_name,
-				 ".text.ftrace_trampoline"))
-			tramp = sechdrs + i;
 		else if (sechdrs[i].sh_type == SHT_SYMTAB)
 			syms = (Elf64_Sym *)sechdrs[i].sh_addr;
 	}
@@ -336,15 +335,8 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
 	pltsec->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
 	pltsec->sh_addralign = L1_CACHE_BYTES;
 	pltsec->sh_size = (core_plts  + 1) * sizeof(struct plt_entry);
-	mod->arch.core.plt_num_entries = 0;
+	mod->arch.core.plt_num_entries = ftrace_plts;
 	mod->arch.core.plt_max_entries = core_plts;
 
-	if (tramp) {
-		tramp->sh_type = SHT_NOBITS;
-		tramp->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
-		tramp->sh_addralign = __alignof__(struct plt_entry);
-		tramp->sh_size = NR_FTRACE_PLTS * sizeof(struct plt_entry);
-	}
-
 	return 0;
 }
diff --git a/arch/arm64/kernel/module.c b/arch/arm64/kernel/module.c
index 6e5b488a219e..620c2b70115e 100644
--- a/arch/arm64/kernel/module.c
+++ b/arch/arm64/kernel/module.c
@@ -454,14 +454,7 @@ static int module_init_ftrace_plt(const Elf_Ehdr *hdr,
 				  struct module *mod)
 {
 #if defined(CONFIG_DYNAMIC_FTRACE)
-	const Elf_Shdr *s;
-	struct plt_entry *plts;
-
-	s = find_section(hdr, sechdrs, ".text.ftrace_trampoline");
-	if (!s)
-		return -ENOEXEC;
-
-	plts = (void *)s->sh_addr;
+	struct plt_entry *plts = (void *)sechdrs[mod->arch.core.plt_shndx].sh_addr;
 
 	__init_plt(&plts[FTRACE_PLT_IDX], FTRACE_ADDR);
 
-- 
2.55.0.860.g4b6b3295ed-goog