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arm64: module/ftrace: intialize PLT at load time

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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@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: 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>
This commit is contained in:
Mark Rutland
2019-10-17 15:26:38 +01:00
parent bd8b21d3dd
commit f1a54ae9af
2 changed files with 35 additions and 52 deletions
Download Patch File Download Diff File Expand all files Collapse all files

55
arch/arm64/kernel/ftrace.c
View File

@@ -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 */