The previous change documents that cleanup_return_instances() can't
always detect the dead frames, the stack can grow. But there is one
special case which imho worth fixing: arch_uretprobe_is_alive() can
return true when the stack didn't actually grow, but the next "call"
insn uses the already invalidated frame.
Test-case:
#include <stdio.h>
#include <setjmp.h>
jmp_buf jmp;
int nr = 1024;
void func_2(void)
{
if (--nr == 0)
return;
longjmp(jmp, 1);
}
void func_1(void)
{
setjmp(jmp);
func_2();
}
int main(void)
{
func_1();
return 0;
}
If you ret-probe func_1() and func_2() prepare_uretprobe() hits the
MAX_URETPROBE_DEPTH limit and "return" from func_2() is not reported.
When we know that the new call is not chained, we can do the more
strict check. In this case "sp" points to the new ret-addr, so every
frame which uses the same "sp" must be dead. The only complication is
that arch_uretprobe_is_alive() needs to know was it chained or not, so
we add the new RP_CHECK_CHAIN_CALL enum and change prepare_uretprobe()
to pass RP_CHECK_CALL only if !chained.
Note: arch_uretprobe_is_alive() could also re-read *sp and check if
this word is still trampoline_vaddr. This could obviously improve the
logic, but I would like to avoid another copy_from_user() especially
in the case when we can't avoid the false "alive == T" positives.
Signed-off-by: Oleg Nesterov <[email protected]>
---
arch/x86/kernel/uprobes.c | 5 ++++-
include/linux/uprobes.h | 1 +
kernel/events/uprobes.c | 14 +++++++-------
3 files changed, 12 insertions(+), 8 deletions(-)
diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index 67eb168..a5c59f2 100644
--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -997,5 +997,8 @@ arch_uretprobe_hijack_return_addr(unsigned long
trampoline_vaddr, struct pt_regs
bool arch_uretprobe_is_alive(struct return_instance *ret, enum rp_check ctx,
struct pt_regs *regs)
{
- return regs->sp <= ret->stack;
+ if (ctx == RP_CHECK_CALL) /* sp was just decremented by "call" insn */
+ return regs->sp < ret->stack;
+ else
+ return regs->sp <= ret->stack;
}
diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
index c0a5402..0bdc72f 100644
--- a/include/linux/uprobes.h
+++ b/include/linux/uprobes.h
@@ -104,6 +104,7 @@ struct return_instance {
enum rp_check {
RP_CHECK_CALL,
+ RP_CHECK_CHAIN_CALL,
RP_CHECK_RET,
};
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index df5661a..0f370ef 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -1511,10 +1511,11 @@ static unsigned long get_trampoline_vaddr(void)
return trampoline_vaddr;
}
-static void cleanup_return_instances(struct uprobe_task *utask, struct pt_regs
*regs)
+static void cleanup_return_instances(struct uprobe_task *utask, bool chained,
+ struct pt_regs *regs)
{
struct return_instance *ri = utask->return_instances;
- enum rp_check ctx = RP_CHECK_CALL;
+ enum rp_check ctx = chained ? RP_CHECK_CHAIN_CALL : RP_CHECK_CALL;
while (ri && !arch_uretprobe_is_alive(ri, ctx, regs)) {
ri = free_ret_instance(ri);
@@ -1528,7 +1529,7 @@ static void prepare_uretprobe(struct uprobe *uprobe,
struct pt_regs *regs)
struct return_instance *ri;
struct uprobe_task *utask;
unsigned long orig_ret_vaddr, trampoline_vaddr;
- bool chained = false;
+ bool chained;
if (!get_xol_area())
return;
@@ -1554,14 +1555,15 @@ static void prepare_uretprobe(struct uprobe *uprobe,
struct pt_regs *regs)
goto fail;
/* drop the entries invalidated by longjmp() */
- cleanup_return_instances(utask, regs);
+ chained = (orig_ret_vaddr == trampoline_vaddr);
+ cleanup_return_instances(utask, chained, regs);
/*
* We don't want to keep trampoline address in stack, rather keep the
* original return address of first caller thru all the consequent
* instances. This also makes breakpoint unwrapping easier.
*/
- if (orig_ret_vaddr == trampoline_vaddr) {
+ if (chained) {
if (!utask->return_instances) {
/*
* This situation is not possible. Likely we have an
@@ -1570,8 +1572,6 @@ static void prepare_uretprobe(struct uprobe *uprobe,
struct pt_regs *regs)
uprobe_warn(current, "handle tail call");
goto fail;
}
-
- chained = true;
orig_ret_vaddr = utask->return_instances->orig_ret_vaddr;
}
--
1.5.5.1
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