[PATCH] kprobes: Reduce preempt disable scope in check_kprobe_access_safe()
STALE678d
LANDED
From: Thomas Weißschuh <hidden>
Date: 2024-11-21 07:39:04
Also in:
linux-rt-devel, lkml
Subsystem:
kprobes, the rest · Maintainers:
Naveen N Rao, "David S. Miller", Masami Hiramatsu, Linus Torvalds
Landed in mainline as 1703abb4dead on 2025-01-10.
Commit a189d0350f387 ("kprobes: disable preempt for module_text_address() and kernel_text_address()")
introduced a preempt_disable() region to protect against concurrent
module unloading. However this region also includes the call to
jump_label_text_reserved() which takes a long time;
up to 400us, iterating over approx 6000 jump tables.
The scope protected by preempt_disable() is largen than necessary.
core_kernel_text() does not need to be protected as it does not interact
with module code at all.
Only the scope from __module_text_address() to try_module_get() needs to
be protected.
By limiting the critical section to __module_text_address() and
try_module_get() the function responsible for the latency spike remains
preemptible.
This works fine even when !CONFIG_MODULES as in that case
try_module_get() will always return true and that block can be optimized
away.
Limit the critical section to __module_text_address() and
try_module_get(). Use guard(preempt)() for easier error handling.
While at it also remove a spurious *probed_mod = NULL in an error
path. On errors the output parameter is never inspected by the caller.
Some error paths were clearing the parameters, some didn't.
Align them for clarity.
Signed-off-by: Thomas Weißschuh <redacted>
---
kernel/kprobes.c | 24 ++++++++++++------------
1 file changed, 12 insertions(+), 12 deletions(-)
diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index da59c68df8412c4662d39d3e286d516e8cee9a69..369020170e832a34ea9c05eda6693bded02ec505 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c@@ -39,6 +39,7 @@ #include <linux/static_call.h> #include <linux/perf_event.h> #include <linux/execmem.h> +#include <linux/cleanup.h> #include <asm/sections.h> #include <asm/cacheflush.h>
@@ -1570,16 +1571,25 @@ static int check_kprobe_address_safe(struct kprobe *p, if (ret) return ret; jump_label_lock(); - preempt_disable(); /* Ensure the address is in a text area, and find a module if exists. */ *probed_mod = NULL; if (!core_kernel_text((unsigned long) p->addr)) { + guard(preempt)(); *probed_mod = __module_text_address((unsigned long) p->addr); if (!(*probed_mod)) { ret = -EINVAL; goto out; } + + /* + * We must hold a refcount of the probed module while updating + * its code to prohibit unexpected unloading. + */ + if (unlikely(!try_module_get(*probed_mod))) { + ret = -ENOENT; + goto out; + } } /* Ensure it is not in reserved area. */ if (in_gate_area_no_mm((unsigned long) p->addr) ||
@@ -1588,21 +1598,13 @@ static int check_kprobe_address_safe(struct kprobe *p, static_call_text_reserved(p->addr, p->addr) || find_bug((unsigned long)p->addr) || is_cfi_preamble_symbol((unsigned long)p->addr)) { + module_put(*probed_mod); ret = -EINVAL; goto out; } /* Get module refcount and reject __init functions for loaded modules. */ if (IS_ENABLED(CONFIG_MODULES) && *probed_mod) { - /* - * We must hold a refcount of the probed module while updating - * its code to prohibit unexpected unloading. - */ - if (unlikely(!try_module_get(*probed_mod))) { - ret = -ENOENT; - goto out; - } - /* * If the module freed '.init.text', we couldn't insert * kprobes in there.
@@ -1610,13 +1612,11 @@ static int check_kprobe_address_safe(struct kprobe *p, if (within_module_init((unsigned long)p->addr, *probed_mod) && !module_is_coming(*probed_mod)) { module_put(*probed_mod); - *probed_mod = NULL; ret = -ENOENT; } } out: - preempt_enable(); jump_label_unlock(); return ret;
--- base-commit: adc218676eef25575469234709c2d87185ca223a change-id: 20241120-kprobes-preempt-09826f5e5d9c Best regards, -- Thomas Weißschuh [off-list ref]