linux-kernel - [PATCH 2/4] arm64/ptrace: introduce orig_x7 in the user_pt

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Message-Id: <20210322225053.428615-3-avagin@gmail.com>
Date:   Mon, 22 Mar 2021 15:50:51 -0700
From:   Andrei Vagin <avagin@...il.com>
To:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Andrei Vagin <avagin@...il.com>,
        Dave Martin <Dave.Martin@....com>,
        Keno Fischer <keno@...iacomputing.com>
Subject: [PATCH 2/4] arm64/ptrace: introduce orig_x7 in the user_pt_regs structure

We have some ABI weirdness in the way that we handle syscall exit stops
because we indicate whether or not the stop has been signalled from
syscall entry or syscall exit by clobbering a general purpose register
x7 in the tracee and restoring its old value after the stop.

This behavior was inherited from ARM and it isn't common for other
architectures. Now, we have PTRACE_GET_SYSCALL_INFO that gives all
required information about system calls, so the hack with clobbering
registers isn't needed anymore.

This change instroduces orig_x7 in the user_pt_regs structure that will
contains an origin value of the x7 register if the tracee is stopped in
a system call..

Signed-off-by: Andrei Vagin <avagin@...il.com>
---
 arch/arm64/include/asm/ptrace.h      |  1 +
 arch/arm64/include/uapi/asm/ptrace.h |  1 +
 arch/arm64/kernel/ptrace.c           | 18 ++++++++++++------
 3 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
index d4cdf98ac003..1008f0fbc5ea 100644
--- a/arch/arm64/include/asm/ptrace.h
+++ b/arch/arm64/include/asm/ptrace.h
@@ -184,6 +184,7 @@ struct pt_regs {
 			u64 pc;
 			u64 pstate;
 			u64 orig_x0;
+			u64 orig_x7;
 		};
 	};
 #ifdef __AARCH64EB__
diff --git a/arch/arm64/include/uapi/asm/ptrace.h b/arch/arm64/include/uapi/asm/ptrace.h
index 3c118c5b0893..be7583ff5f4d 100644
--- a/arch/arm64/include/uapi/asm/ptrace.h
+++ b/arch/arm64/include/uapi/asm/ptrace.h
@@ -91,6 +91,7 @@ struct user_pt_regs {
 	__u64		pc;
 	__u64		pstate;
 	__u64		orig_x0;
+	__u64		orig_x7;
 };
 
 struct user_fpsimd_state {
diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 170f42fd6101..1ed5b4aa986b 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -1750,7 +1750,7 @@ static void tracehook_report_syscall(struct pt_regs *regs,
 				     enum ptrace_syscall_dir dir)
 {
 	int regno;
-	unsigned long saved_reg;
+	u64 _saved_reg, *saved_reg;
 
 	/*
 	 * We have some ABI weirdness here in the way that we handle syscall
@@ -1768,19 +1768,25 @@ static void tracehook_report_syscall(struct pt_regs *regs,
 	 * - Syscall stops behave differently to seccomp and pseudo-step traps
 	 *   (the latter do not nobble any registers).
 	 */
-	regno = (is_compat_task() ? 12 : 7);
-	saved_reg = regs->regs[regno];
+	if (is_compat_task()) {
+		regno = 12;
+		saved_reg = &_saved_reg;
+	} else {
+		regno = 7;
+		saved_reg = &regs->orig_x7;
+	}
+	*saved_reg = regs->regs[regno];
 	regs->regs[regno] = dir;
 
 	if (dir == PTRACE_SYSCALL_ENTER) {
 		if (tracehook_report_syscall_entry(regs))
 			forget_syscall(regs);
-		regs->regs[regno] = saved_reg;
+		regs->regs[regno] = *saved_reg;
 	} else if (!test_thread_flag(TIF_SINGLESTEP)) {
 		tracehook_report_syscall_exit(regs, 0);
-		regs->regs[regno] = saved_reg;
+		regs->regs[regno] = *saved_reg;
 	} else {
-		regs->regs[regno] = saved_reg;
+		regs->regs[regno] = *saved_reg;
 
 		/*
 		 * Signal a pseudo-step exception since we are stepping but
-- 
2.29.2