linux-kernel - [tip: perf/urgent] uprobes: Change handle_swbp() to send SIGTRAP with si_code=SI

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Date:   Fri, 24 Jul 2020 14:07:41 -0000
From:   "tip-bot2 for Oleg Nesterov" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Aaron Merey <amerey@...hat.com>, Oleg Nesterov <oleg@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        stable@...r.kernel.org, x86 <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: [tip: perf/urgent] uprobes: Change handle_swbp() to send SIGTRAP with
 si_code=SI_KERNEL, to fix GDB regression

The following commit has been merged into the perf/urgent branch of tip:

Commit-ID:     fe5ed7ab99c656bd2f5b79b49df0e9ebf2cead8a
Gitweb:        https://git.kernel.org/tip/fe5ed7ab99c656bd2f5b79b49df0e9ebf2cead8a
Author:        Oleg Nesterov <oleg@...hat.com>
AuthorDate:    Thu, 23 Jul 2020 17:44:20 +02:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Fri, 24 Jul 2020 15:38:37 +02:00

uprobes: Change handle_swbp() to send SIGTRAP with si_code=SI_KERNEL, to fix GDB regression

If a tracee is uprobed and it hits int3 inserted by debugger, handle_swbp()
does send_sig(SIGTRAP, current, 0) which means si_code == SI_USER. This used
to work when this code was written, but then GDB started to validate si_code
and now it simply can't use breakpoints if the tracee has an active uprobe:

	# cat test.c
	void unused_func(void)
	{
	}
	int main(void)
	{
		return 0;
	}

	# gcc -g test.c -o test
	# perf probe -x ./test -a unused_func
	# perf record -e probe_test:unused_func gdb ./test -ex run
	GNU gdb (GDB) 10.0.50.20200714-git
	...
	Program received signal SIGTRAP, Trace/breakpoint trap.
	0x00007ffff7ddf909 in dl_main () from /lib64/ld-linux-x86-64.so.2
	(gdb)

The tracee hits the internal breakpoint inserted by GDB to monitor shared
library events but GDB misinterprets this SIGTRAP and reports a signal.

Change handle_swbp() to use force_sig(SIGTRAP), this matches do_int3_user()
and fixes the problem.

This is the minimal fix for -stable, arch/x86/kernel/uprobes.c is equally
wrong; it should use send_sigtrap(TRAP_TRACE) instead of send_sig(SIGTRAP),
but this doesn't confuse GDB and needs another x86-specific patch.

Reported-by: Aaron Merey <amerey@...hat.com>
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Reviewed-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc: stable@...r.kernel.org
Link: https://lore.kernel.org/r/20200723154420.GA32043@redhat.com
---
 kernel/events/uprobes.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index bb08628..5f8b0c5 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -2199,7 +2199,7 @@ static void handle_swbp(struct pt_regs *regs)
 	if (!uprobe) {
 		if (is_swbp > 0) {
 			/* No matching uprobe; signal SIGTRAP. */
-			send_sig(SIGTRAP, current, 0);
+			force_sig(SIGTRAP);
 		} else {
 			/*
 			 * Either we raced with uprobe_unregister() or we can't