Re: [PATCH bpf-next 1/3] s390/bpf: Add orig_gpr2 to user_pt_regs

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Hi Heiko,

Heiko Carstens wrote:
On Wed, Feb 02, 2022 at 12:41:58AM +0100, Ilya Leoshkevich wrote:
user_pt_regs is used by eBPF in order to access userspace registers -
see commit 466698e654e8 ("s390/bpf: correct broken uapi for
BPF_PROG_TYPE_PERF_EVENT program type"). In order to access the first
syscall argument from eBPF programs, we need to export orig_gpr2.

Signed-off-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
---
 arch/s390/include/asm/ptrace.h      | 2 +-
 arch/s390/include/uapi/asm/ptrace.h | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/s390/include/asm/ptrace.h b/arch/s390/include/asm/ptrace.h
index 4ffa8e7f0ed3..c8698e643904 100644
--- a/arch/s390/include/asm/ptrace.h
+++ b/arch/s390/include/asm/ptrace.h
@@ -83,9 +83,9 @@ struct pt_regs {
 			unsigned long args[1];
 			psw_t psw;
 			unsigned long gprs[NUM_GPRS];
+			unsigned long orig_gpr2;
 		};
 	};
-	unsigned long orig_gpr2;
 	union {
 		struct {
 			unsigned int int_code;
diff --git a/arch/s390/include/uapi/asm/ptrace.h b/arch/s390/include/uapi/asm/ptrace.h
index ad64d673b5e6..b3dec603f507 100644
--- a/arch/s390/include/uapi/asm/ptrace.h
+++ b/arch/s390/include/uapi/asm/ptrace.h
@@ -295,6 +295,7 @@ typedef struct {
 	unsigned long args[1];
 	psw_t psw;
 	unsigned long gprs[NUM_GPRS];
+	unsigned long orig_gpr2;
 } user_pt_regs;

Isn't this broken on nearly all architectures? I just checked powerpc,
arm64, and riscv. While powerpc seems to mirror pt_regs as user_pt_regs,
and therefore exports orig_gpr3, the bpf macros still seem to access the
wrong location to access the first syscall parameter(?).

On powerpc, gpr[3] continues to be valid on syscall entry (so this test passes on powerpc), though orig_gpr3 will remain valid throughout.

I will submit a patch to use orig_gpr3 on powerpc.


Thanks!
- Naveen





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