arm64: implement ftrace with regs

This patch implements FTRACE_WITH_REGS for arm64, which allows a traced
function's arguments (and some other registers) to be captured into a
struct pt_regs, allowing these to be inspected and/or modified. This is
a building block for live-patching, where a function's arguments may be
forwarded to another function. This is also necessary to enable ftrace
and in-kernel pointer authentication at the same time, as it allows the
LR value to be captured and adjusted prior to signing.

Using GCC's -fpatchable-function-entry=N option, we can have the
compiler insert a configurable number of NOPs between the function entry
point and the usual prologue. This also ensures functions are AAPCS
compliant (e.g. disabling inter-procedural register allocation).

For example, with -fpatchable-function-entry=2, GCC 8.1.0 compiles the
following:

| unsigned long bar(void);
|
| unsigned long foo(void)
| {
|         return bar() + 1;
| }

... to:

| <foo>:
|         nop
|         nop
|         stp     x29, x30, [sp, #-16]!
|         mov     x29, sp
|         bl      0 <bar>
|         add     x0, x0, #0x1
|         ldp     x29, x30, [sp], #16
|         ret

This patch builds the kernel with -fpatchable-function-entry=2,
prefixing each function with two NOPs. To trace a function, we replace
these NOPs with a sequence that saves the LR into a GPR, then calls an
ftrace entry assembly function which saves this and other relevant
registers:

| mov	x9, x30
| bl	<ftrace-entry>

Since patchable functions are AAPCS compliant (and the kernel does not
use x18 as a platform register), x9-x18 can be safely clobbered in the
patched sequence and the ftrace entry code.

There are now two ftrace entry functions, ftrace_regs_entry (which saves
all GPRs), and ftrace_entry (which saves the bare minimum). A PLT is
allocated for each within modules.

Signed-off-by: Torsten Duwe <duwe@suse.de>
[Mark: rework asm, comments, PLTs, initialization, commit message]
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Amit Daniel Kachhap <amit.kachhap@arm.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Torsten Duwe <duwe@suse.de>
Tested-by: Amit Daniel Kachhap <amit.kachhap@arm.com>
Tested-by: Torsten Duwe <duwe@suse.de>
Cc: AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Julien Thierry <jthierry@redhat.com>
Cc: Will Deacon <will@kernel.org>

1 files changed, 14 insertions, 4 deletions

diff --git a/arch/arm64/kernel/module.c b/arch/arm64/kernel/module.c
index d0692ecb99bb..1cd1a4d0ed30 100644
--- a/arch/arm64/kernel/module.c
+++ b/arch/arm64/kernel/module.c
@@ -486,21 +486,31 @@ static const Elf_Shdr *find_section(const Elf_Ehdr *hdr,
 	return NULL;
 }
 
+static inline void __init_plt(struct plt_entry *plt, unsigned long addr)
+{
+	*plt = get_plt_entry(addr, plt);
+}
+
 static int module_init_ftrace_plt(const Elf_Ehdr *hdr,
 				  const Elf_Shdr *sechdrs,
 				  struct module *mod)
 {
 #if defined(CONFIG_ARM64_MODULE_PLTS) && defined(CONFIG_DYNAMIC_FTRACE)
 	const Elf_Shdr *s;
-	struct plt_entry *plt;
+	struct plt_entry *plts;
 
 	s = find_section(hdr, sechdrs, ".text.ftrace_trampoline");
 	if (!s)
 		return -ENOEXEC;
 
-	plt = (void *)s->sh_addr;
-	*plt = get_plt_entry(FTRACE_ADDR, plt);
-	mod->arch.ftrace_trampoline = plt;
+	plts = (void *)s->sh_addr;
+
+	__init_plt(&plts[FTRACE_PLT_IDX], FTRACE_ADDR);
+
+	if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_REGS))
+		__init_plt(&plts[FTRACE_REGS_PLT_IDX], FTRACE_REGS_ADDR);
+
+	mod->arch.ftrace_trampolines = plts;
 #endif
 	return 0;
 }