[PATCH] powerpc: optprobes: fix TOC handling in optprobes trampoline

Naveen N. Rao naveen.n.rao at linux.vnet.ibm.com
Wed Feb 22 02:00:47 AEDT 2017

Optprobes on powerpc is limited to kernel text area. We decided to also
optimize kretprobe_trampoline since that is also in kernel text area.
However,we failed to take into consideration the fact that the same
trampoline is also used to catch function returns from kernel modules.
As an example:

    $ sudo modprobe kobject-example
    $ sudo bash -c "echo 'r foo_show+8' > /sys/kernel/debug/tracing/kprobe_events"
    $ sudo bash -c "echo 1 > /sys/kernel/debug/tracing/events/kprobes/enable"
    $ sudo cat /sys/kernel/debug/kprobes/list
    c000000000041350  k  kretprobe_trampoline+0x0    [OPTIMIZED]
    d000000000e00200  r  foo_show+0x8  kobject_example
    $ cat /sys/kernel/kobject_example/foo
    Segmentation fault

With the below (trimmed) splat in dmesg:

    [70646.248029] Unable to handle kernel paging request for data at address 0xfec40000
    [70646.248730] Faulting instruction address: 0xc000000000041540
    [70646.249210] Oops: Kernel access of bad area, sig: 11 [#1]
    [70646.259635] NIP [c000000000041540] optimized_callback+0x70/0xe0
    [70646.259962] LR [c000000000041e60] optinsn_slot+0xf8/0x10000
    [70646.260268] Call Trace:
    [70646.260583] [c0000000c7327850] [c000000000289af4] alloc_set_pte+0x1c4/0x860 (unreliable)
    [70646.260910] [c0000000c7327890] [c000000000041e60] optinsn_slot+0xf8/0x10000
    [70646.261223] --- interrupt: 700 at 0xc0000000c7327a80
		       LR = kretprobe_trampoline+0x0/0x10
    [70646.261849] [c0000000c7327ba0] [c0000000003a30d4] sysfs_kf_seq_show+0x104/0x1d0
    [70646.262135] [c0000000c7327bf0] [c0000000003a0bb4] kernfs_seq_show+0x44/0x60
    [70646.264211] [c0000000c7327c10] [c000000000330578] seq_read+0xf8/0x560
    [70646.265142] [c0000000c7327cb0] [c0000000003a1e64] kernfs_fop_read+0x194/0x260
    [70646.266070] [c0000000c7327d00] [c0000000002f9954] __vfs_read+0x44/0x1a0
    [70646.266977] [c0000000c7327d90] [c0000000002fb4cc] vfs_read+0xbc/0x1b0
    [70646.267860] [c0000000c7327de0] [c0000000002fd138] SyS_read+0x68/0x110
    [70646.268701] [c0000000c7327e30] [c00000000000b8e0] system_call+0x38/0xfc

Fix this by loading up the kernel TOC before calling into the kernel.
The original TOC gets restored as part of the usual pt_regs restore.

Signed-off-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com>
 arch/powerpc/kernel/optprobes_head.S | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/powerpc/kernel/optprobes_head.S b/arch/powerpc/kernel/optprobes_head.S
index 53e429b5a29d..bf28188f308c 100644
--- a/arch/powerpc/kernel/optprobes_head.S
+++ b/arch/powerpc/kernel/optprobes_head.S
@@ -65,6 +65,13 @@ optprobe_template_entry:
 	mfdsisr	r5
 	std	r5,_DSISR(r1)
+	/*
+	 * We may get here from a module, so load the kernel TOC in r2.
+	 * The original TOC gets restored when pt_regs is restored
+	 *  further below.
+	 */
+	ld	r2,PACATOC(r13)
 	.global optprobe_template_op_address

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