Re: [PATCH v2] ARM:unwind:fix unwind abort for uleb128 case

From: AngeloGioacchino Del Regno
Date: Thu Apr 13 2023 - 09:28:45 EST


Il 13/04/23 09:34, Haibo Li ha scritto:
When unwind instruction is 0xb2,the subsequent instructions
are uleb128 bytes.
For now,it uses only the first uleb128 byte in code.

For vsp increments of 0x204~0x400,use one uleb128 byte like below:
0xc06a00e4 <unwind_test_work>: 0x80b27fac
Compact model index: 0
0xb2 0x7f vsp = vsp + 1024
0xac pop {r4, r5, r6, r7, r8, r14}

For vsp increments larger than 0x400,use two uleb128 bytes like below:
0xc06a00e4 <unwind_test_work>: @0xc0cc9e0c
Compact model index: 1
0xb2 0x81 0x01 vsp = vsp + 1032
0xac pop {r4, r5, r6, r7, r8, r14}
The unwind works well since the decoded uleb128 byte is also 0x81.

For vsp increments larger than 0x600,use two uleb128 bytes like below:
0xc06a00e4 <unwind_test_work>: @0xc0cc9e0c
Compact model index: 1
0xb2 0x81 0x02 vsp = vsp + 1544
0xac pop {r4, r5, r6, r7, r8, r14}
In this case,the decoded uleb128 result is 0x101(vsp=0x204+(0x101<<2)).
While the uleb128 used in code is 0x81(vsp=0x204+(0x81<<2)).
The unwind aborts at this frame since it gets incorrect vsp.

To fix this,add uleb128 decode to cover all the above case.

Signed-off-by: Haibo Li <haibo.li@xxxxxxxxxxxx>
---
v2:
- As Linus Walleij and Alexandre Mergnat suggested,add comments for unwind_decode_uleb128
- As Alexandre Mergnat suggested,change variables declaration in Alphabetical order
---
arch/arm/kernel/unwind.c | 25 ++++++++++++++++++++++++-
1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/arch/arm/kernel/unwind.c b/arch/arm/kernel/unwind.c
index 53be7ea6181b..f37e55fcf81d 100644
--- a/arch/arm/kernel/unwind.c
+++ b/arch/arm/kernel/unwind.c
@@ -308,6 +308,29 @@ static int unwind_exec_pop_subset_r0_to_r3(struct unwind_ctrl_block *ctrl,
return URC_OK;
}
+static unsigned long unwind_decode_uleb128(struct unwind_ctrl_block *ctrl)
+{
+ unsigned long bytes = 0;
+ unsigned long insn;
+ unsigned long result = 0;
+
+ /* unwind_get_byte() will advance ctrl one instruction at a time,
+ * we loop until we get an instruction byte where bit 7 is not set.
+ * Note:It decodes max 4 bytes to output 28bits data.
+ * 28bits data(0xfffffff) covers vsp increments of 1073742336.
+ * It is sufficent for unwinding stack.
+ */

/*
* unwind_get_byte() will advance `ctrl` one instruction at a time, so
* loop until we get an instruction byte where bit 7 is not set.
*
* Note: This decodes a maximum of 4 bytes to output 28 bits data where
* max is 0xfffffff: that will cover a vsp increment of 1073742336, hence
* it is sufficient for unwinding the stack.
*/

+ do {
+ insn = unwind_get_byte(ctrl);
+ result |= (insn & 0x7f) << (bytes * 7);
+ bytes++;

also, I would do ...

} while (!!(insn & 0x80) && bytes != sizeof(result));

...compressing the code and not creating any human readability concern.

after which, you can get my

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@xxxxxxxxxxxxx>