[PATCH v2] csky: Fix a4/a5 restoration in syscall trace path

[hlsong <pgeorge8929@gmail.com>]

From: hlsong89 <pgeorge8929@gmail.com>

The syscall trace path reloads syscall arguments from pt_regs before
calling the syscall handler. On C-SKY ABIv2, the 5th and 6th syscall
arguments are prepared as stack arguments before invoking syscallid.

The current code adjusts sp before loading LSAVE_A4 and LSAVE_A5. Since
those offsets are relative to the original pt_regs base, loading them
after changing sp fetches the wrong slots. As a result, traced syscalls
that use the 5th or 6th argument may receive corrupted arguments.

This is visible with mmap2(), which takes six arguments. A small
PTRACE_SYSCALL reproducer opens a file and maps one page with:

  mmap(NULL, 4096, PROT_READ | PROT_EXEC, MAP_PRIVATE, fd, 0)

Before the fix, the traced child fails the mmap and exits with 12.
After the fix, the mapping succeeds and the child exits with 0.

Fix the trace path by loading a4/a5 from pt_regs before changing sp.

Tested on: ck860f, linux-4.19.15, C-SKY abiv2

Suggested-by: Guo Ren <guoren@kernel.org>
Signed-off-by: hlsong89 <pgeorge8929@gmail.com>
---
Changes in v2:
- Use Guo Ren's suggested approach to handle the ABIv2 stack arguments.
- Tested with the ptrace+mmap reproducer.

 arch/csky/kernel/entry.S | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/csky/kernel/entry.S b/arch/csky/kernel/entry.S
index c68cdcc76..3261f46f2 100644
--- a/arch/csky/kernel/entry.S
+++ b/arch/csky/kernel/entry.S
@@ -93,11 +93,11 @@ csky_syscall_trace:
 	ldw	a2, (sp, LSAVE_A2)
 	ldw	a3, (sp, LSAVE_A3)
 #if defined(__CSKYABIV2__)
-	subi	sp, 8
 	ldw	r9, (sp, LSAVE_A4)
+	ldw	r10, (sp, LSAVE_A5)
+	subi	sp, 8
 	stw	r9, (sp, 0x0)
-	ldw	r9, (sp, LSAVE_A5)
-	stw	r9, (sp, 0x4)
+	stw	r10, (sp, 0x4)
 	jsr	syscallid                     /* Do system call */
 	addi	sp, 8
 #else

base-commit: 5200f5f493f79f14bbdc349e402a40dfb32f23c8
-- 
2.25.1