[RFC PATCH 07/12] powerpc: Use orig_gpr3 in syscall_get_arguments()

[Michael Ellerman]

Currently syscall_get_arguments() is used by syscall tracepoints, and
collect_syscall() which is used in some debugging as well as
/proc/pid/syscall.

The current implementation just copies regs->gpr[3 .. 5] out, which is
fine for all the current use cases.

When we enable seccomp filter, that will also start using
syscall_get_arguments(). However for seccomp filter we want to use r3
as the return value of the syscall, and orig_gpr3 as the first
parameter. This will allow seccomp to modify the return value in r3.

To support this we need to modify syscall_get_arguments() to return
orig_gpr3 instead of r3. This is safe for all uses because orig_gpr3
always contains the r3 value that was passed to the syscall. We store it
in the syscall entry path and never modify it.

Update syscall_set_arguments() while we're here, even though it's never
used.

Signed-off-by: Michael Ellerman <m...@ellerman.id.au>
---
 arch/powerpc/include/asm/syscall.h | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/include/asm/syscall.h 
b/arch/powerpc/include/asm/syscall.h
index 403e2303fe18..8d79a87c0511 100644
--- a/arch/powerpc/include/asm/syscall.h
+++ b/arch/powerpc/include/asm/syscall.h
@@ -64,7 +64,7 @@ static inline void syscall_get_arguments(struct task_struct 
*task,
                                         unsigned int i, unsigned int n,
                                         unsigned long *args)
 {
-       unsigned long mask = -1UL;
+       unsigned long val, mask = -1UL;
 
        BUG_ON(i + n > 6);
 
@@ -72,8 +72,14 @@ static inline void syscall_get_arguments(struct task_struct 
*task,
        if (test_tsk_thread_flag(task, TIF_32BIT))
                mask = 0xffffffff;
 #endif
-       while (n--)
-               args[n] = regs->gpr[3 + i + n] & mask;
+       while (n--) {
+               if (n == 0 && i == 0)
+                       val = regs->orig_gpr3;
+               else
+                       val = regs->gpr[3 + i + n];
+
+               args[n] = val & mask;
+       }
 }
 
 static inline void syscall_set_arguments(struct task_struct *task,
@@ -83,6 +89,10 @@ static inline void syscall_set_arguments(struct task_struct 
*task,
 {
        BUG_ON(i + n > 6);
        memcpy(&regs->gpr[3 + i], args, n * sizeof(args[0]));
+
+       /* Also copy the first argument into orig_gpr3 */
+       if (i == 0 && n > 0)
+               regs->orig_gpr3 = args[0];
 }
 
 static inline int syscall_get_arch(void)
-- 
2.1.0

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