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Date: Sat, 2 Nov 2019 17:50:02 +0530
From: Amit Daniel Kachhap <amit.kachhap@....com>
To: Mark Rutland <mark.rutland@....com>,
linux-arm-kernel@...ts.infradead.org
Cc: linux-kernel@...r.kernel.org, catalin.marinas@....com,
deller@....de, duwe@...e.de, James.Bottomley@...senPartnership.com,
james.morse@....com, jeyu@...nel.org, jpoimboe@...hat.com,
jthierry@...hat.com, linux-parisc@...r.kernel.org,
mingo@...hat.com, peterz@...radead.org, rostedt@...dmis.org,
svens@...ckframe.org, takahiro.akashi@...aro.org, will@...nel.org
Subject: Re: [PATCHv2 4/8] arm64: module/ftrace: intialize PLT at load time
Hi,
On 10/29/19 10:28 PM, Mark Rutland wrote:
> 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: Mark Rutland <mark.rutland@....com>
> Reviewed-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: James Morse <james.morse@....com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Will Deacon <will@...nel.org>
> ---
> arch/arm64/kernel/ftrace.c | 55 ++++++++++++----------------------------------
> arch/arm64/kernel/module.c | 32 +++++++++++++++++----------
> 2 files changed, 35 insertions(+), 52 deletions(-)
>
> diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
> index 06e56b470315..822718eafdb4 100644
> --- a/arch/arm64/kernel/ftrace.c
> +++ b/arch/arm64/kernel/ftrace.c
> @@ -73,10 +73,22 @@ 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
> * range of an ordinary relative 'bl' opcode. In this case, we
> @@ -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 */
> diff --git a/arch/arm64/kernel/module.c b/arch/arm64/kernel/module.c
> index 763a86d52fef..5f5bc3b94da7 100644
> --- a/arch/arm64/kernel/module.c
> +++ b/arch/arm64/kernel/module.c
> @@ -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_init_ftrace_plt(const Elf_Ehdr *hdr,
> + const Elf_Shdr *sechdrs,
> + struct module *mod)
I think this function can be made static as it is not used anywhere.
Thanks,
Amit Daniel
> +{
> +#if defined(CONFIG_ARM64_MODULE_PLTS) && defined(CONFIG_DYNAMIC_FTRACE)
> + const Elf_Shdr *s;
> + struct plt_entry *plt;
> +
> + 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;
> +#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);
>
> -#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
> -
> - return 0;
> + return module_init_ftrace_plt(hdr, sechdrs, me);
> }
>
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