Commit f1a54ae9 authored by Mark Rutland's avatar Mark Rutland
Browse files

arm64: module/ftrace: intialize PLT at load time 



Currently we lazily-initialize a module's ftrace PLT at runtime when we
install the first ftrace call. To do so we have to apply a number of
sanity checks, transiently mark the module text as RW, and perform an
IPI as part of handling Neoverse-N1 erratum #1542419.

We only expect the ftrace trampoline to point at ftrace_caller() (AKA
FTRACE_ADDR), so let's simplify all of this by intializing the PLT at
module load time, before the module loader marks the module RO and
performs the intial I-cache maintenance for the module.

Thus we can rely on the module having been correctly intialized, and can
simplify the runtime work necessary to install an ftrace call in a
module. This will also allow for the removal of module_disable_ro().

Tested by forcing ftrace_make_call() to use the module PLT, and then
loading up a module after setting up ftrace with:

| echo ":mod:<module-name>" > set_ftrace_filter;
| echo function > current_tracer;
| modprobe <module-name>

Since FTRACE_ADDR is only defined when CONFIG_DYNAMIC_FTRACE is
selected, we wrap its use along with most of module_init_ftrace_plt()
with ifdeffery rather than using IS_ENABLED().

Signed-off-by: default avatarMark Rutland <mark.rutland@arm.com>
Reviewed-by: default avatarAmit Daniel Kachhap <amit.kachhap@arm.com>
Reviewed-by: default avatarArd Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: default avatarTorsten Duwe <duwe@suse.de>
Tested-by: default avatarAmit Daniel Kachhap <amit.kachhap@arm.com>
Tested-by: default avatarTorsten Duwe <duwe@suse.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>
parent bd8b21d3

arch/arm64/kernel/ftrace.c

+14 −41
Original line number Diff line number Diff line
@@ -73,9 +73,21 @@ int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr) 


	if (offset < -SZ_128M || offset >= SZ_128M) {

#ifdef CONFIG_ARM64_MODULE_PLTS

		struct plt_entry trampoline, *dst;

		struct module *mod;



		/*

		 * There is only one ftrace trampoline per module. For now,

		 * this is not a problem since on arm64, all dynamic ftrace

		 * invocations are routed via ftrace_caller(). This will need

		 * to be revisited if support for multiple ftrace entry points

		 * is added in the future, but for now, the pr_err() below

		 * deals with a theoretical issue only.

		 */

		if (addr != FTRACE_ADDR) {

			pr_err("ftrace: far branches to multiple entry points unsupported inside a single module\n");

			return -EINVAL;

		}



		/*

		 * On kernels that support module PLTs, the offset between the

		 * branch instruction and its target may legally exceed the
@@ -93,46 +105,7 @@ int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr) 
		if (WARN_ON(!mod))

			return -EINVAL;



		/*

		 * There is only one ftrace trampoline per module. For now,

		 * this is not a problem since on arm64, all dynamic ftrace

		 * invocations are routed via ftrace_caller(). This will need

		 * to be revisited if support for multiple ftrace entry points

		 * is added in the future, but for now, the pr_err() below

		 * deals with a theoretical issue only.

		 *

		 * Note that PLTs are place relative, and plt_entries_equal()

		 * checks whether they point to the same target. Here, we need

		 * to check if the actual opcodes are in fact identical,

		 * regardless of the offset in memory so use memcmp() instead.

		 */

		dst = mod->arch.ftrace_trampoline;

		trampoline = get_plt_entry(addr, dst);

		if (memcmp(dst, &trampoline, sizeof(trampoline))) {

			if (plt_entry_is_initialized(dst)) {

				pr_err("ftrace: far branches to multiple entry points unsupported inside a single module\n");

				return -EINVAL;

			}



			/* point the trampoline to our ftrace entry point */

			module_disable_ro(mod);

			*dst = trampoline;

			module_enable_ro(mod, true);



			/*

			 * Ensure updated trampoline is visible to instruction

			 * fetch before we patch in the branch. Although the

			 * architecture doesn't require an IPI in this case,

			 * Neoverse-N1 erratum #1542419 does require one

			 * if the TLB maintenance in module_enable_ro() is

			 * skipped due to rodata_enabled. It doesn't seem worth

			 * it to make it conditional given that this is

			 * certainly not a fast-path.

			 */

			flush_icache_range((unsigned long)&dst[0],

					   (unsigned long)&dst[1]);

		}

		addr = (unsigned long)dst;

		addr = (unsigned long)mod->arch.ftrace_trampoline;

#else /* CONFIG_ARM64_MODULE_PLTS */

		return -EINVAL;

#endif /* CONFIG_ARM64_MODULE_PLTS */

arch/arm64/kernel/module.c

+21 −11
Original line number Diff line number Diff line
@@ -9,6 +9,7 @@ 


#include <linux/bitops.h>

#include <linux/elf.h>

#include <linux/ftrace.h>

#include <linux/gfp.h>

#include <linux/kasan.h>

#include <linux/kernel.h>
@@ -485,24 +486,33 @@ static const Elf_Shdr *find_section(const Elf_Ehdr *hdr, 
	return NULL;

}



int module_finalize(const Elf_Ehdr *hdr,

static int module_init_ftrace_plt(const Elf_Ehdr *hdr,

				  const Elf_Shdr *sechdrs,

		    struct module *me)

				  struct module *mod)

{

#if defined(CONFIG_ARM64_MODULE_PLTS) && defined(CONFIG_DYNAMIC_FTRACE)

	const Elf_Shdr *s;

	struct plt_entry *plt;



	s = find_section(hdr, sechdrs, ".altinstructions");

	if (s)

		apply_alternatives_module((void *)s->sh_addr, s->sh_size);



#ifdef CONFIG_ARM64_MODULE_PLTS

	if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE)) {

	s = find_section(hdr, sechdrs, ".text.ftrace_trampoline");

	if (!s)

		return -ENOEXEC;

		me->arch.ftrace_trampoline = (void *)s->sh_addr;

	}

#endif



	plt = (void *)s->sh_addr;

	*plt = get_plt_entry(FTRACE_ADDR, plt);

	mod->arch.ftrace_trampoline = plt;

#endif

	return 0;

}



int module_finalize(const Elf_Ehdr *hdr,

		    const Elf_Shdr *sechdrs,

		    struct module *me)

{

	const Elf_Shdr *s;

	s = find_section(hdr, sechdrs, ".altinstructions");

	if (s)

		apply_alternatives_module((void *)s->sh_addr, s->sh_size);



	return module_init_ftrace_plt(hdr, sechdrs, me);

}

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