[PATCH 2/3] arm64/ptrace: introduce PTRACE_O_ARM64_RAW_REGS

Andrei Vagin
2021-02-01T19:42:53Z

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 (ip/r12 for AArch32, x7 for AArch64) 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 adds the new ptrace option PTRACE_O_ARM64_RAW_REGS. If it
is set, PTRACE_GETREGSET returns values of all registers without
clobbering r12 or x7 and PTRACE_SETREGSE sets all registers even if a
process has been stopped in syscall-enter or syscall-exit.

Signed-off-by: Andrei Vagin <avagin@gmail.com>
---
arch/arm64/include/uapi/asm/ptrace.h | 4 ++
arch/arm64/kernel/ptrace.c | 70 ++++++++++++++++------------
include/linux/ptrace.h | 1 +
include/uapi/linux/ptrace.h | 9 +++-
4 files changed, 52 insertions(+), 32 deletions(-)

diff --git a/arch/arm64/include/uapi/asm/ptrace.h b/arch/arm64/include/uapi/asm/ptrace.h
index 758ae984ff97..465cc9713895 100644
--- a/arch/arm64/include/uapi/asm/ptrace.h
+++ b/arch/arm64/include/uapi/asm/ptrace.h
@@ -109,6 +109,10 @@ struct user_hwdebug_state {
} dbg_regs[16];
};

+#define PTRACE_O_ARM64_RAW_REGS (1 << 28)
+
+#define _PTRACE_O_ARCH_OPTIONS PTRACE_O_ARM64_RAW_REGS
+
/* SVE/FP/SIMD state (NT_ARM_SVE) */

struct user_sve_header {
diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 39da03104528..591a4478ad76 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -565,21 +565,23 @@ static int gpr_get(struct task_struct *target,
unsigned long saved_reg;
int ret;

- /*
- * We have some ABI weirdness here 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 the general
- * purpose register x7.
- */
saved_reg = uregs->regs[7];

- switch (target->ptrace_message) {
- case PTRACE_EVENTMSG_SYSCALL_ENTRY:
- uregs->regs[7] = PTRACE_SYSCALL_ENTER;
- break;
- case PTRACE_EVENTMSG_SYSCALL_EXIT:
- uregs->regs[7] = PTRACE_SYSCALL_EXIT;
- break;
+ if (!(target->ptrace & PT_ARM64_RAW_REGS)) {
+ /*
+ * We have some ABI weirdness here 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 the general purpose register x7.
+ */
+ switch (target->ptrace_message) {
+ case PTRACE_EVENTMSG_SYSCALL_ENTRY:
+ uregs->regs[7] = PTRACE_SYSCALL_ENTER;
+ break;
+ case PTRACE_EVENTMSG_SYSCALL_EXIT:
+ uregs->regs[7] = PTRACE_SYSCALL_EXIT;
+ break;
+ }
}

ret = membuf_write(&to, uregs, sizeof(*uregs));
@@ -600,15 +602,17 @@ static int gpr_set(struct task_struct *target, const struct user_regset *regset,
if (ret)
return ret;

- /*
- * Historically, x7 can't be changed if the stop has been signalled
- * from syscall-enter of syscall-exit.
- */
- switch (target->ptrace_message) {
- case PTRACE_EVENTMSG_SYSCALL_ENTRY:
- case PTRACE_EVENTMSG_SYSCALL_EXIT:
- newregs.regs[7] = task_pt_regs(target)->regs[7];
- break;
+ if (!(target->ptrace & PT_ARM64_RAW_REGS)) {
+ /*
+ * Historically, x7 can't be changed if the stop has been
+ * signalled from syscall-enter of syscall-exit.
+ */
+ switch (target->ptrace_message) {
+ case PTRACE_EVENTMSG_SYSCALL_ENTRY:
+ case PTRACE_EVENTMSG_SYSCALL_EXIT:
+ newregs.regs[7] = task_pt_regs(target)->regs[7];
+ break;
+ }
}

if (!valid_user_regs(&newregs, target))
@@ -1243,6 +1247,8 @@ static inline compat_ulong_t compat_get_user_reg(struct task_struct *task, int i

switch (idx) {
case 12:
+ if (task->ptrace & PT_ARM64_RAW_REGS)
+ return regs->regs[idx];
/*
* We have some ABI weirdness here in the way that we handle
* syscall exit stops because we indicate whether or not the
@@ -1332,15 +1338,17 @@ static int compat_gpr_set(struct task_struct *target,

}

- /*
- * Historically, x12 can't be changed if the stop has been signalled
- * from syscall-enter of syscall-exit.
- */
- switch (target->ptrace_message) {
- case PTRACE_EVENTMSG_SYSCALL_ENTRY:
- case PTRACE_EVENTMSG_SYSCALL_EXIT:
- newregs.regs[12] = task_pt_regs(target)->regs[12];
- break;
+ if (!(target->ptrace & PT_ARM64_RAW_REGS)) {
+ /*
+ * Historically, r12 can't be changed if the stop has been
+ * signalled from syscall-enter of syscall-exit.
+ */
+ switch (target->ptrace_message) {
+ case PTRACE_EVENTMSG_SYSCALL_ENTRY:
+ case PTRACE_EVENTMSG_SYSCALL_EXIT:
+ newregs.regs[12] = task_pt_regs(target)->regs[12];
+ break;
+ }
}

if (valid_user_regs(&newregs.user_regs, target))
diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index 2a9df80ea887..987d6ec5f0ce 100644
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -46,6 +46,7 @@ extern int ptrace_access_vm(struct task_struct *tsk, unsigned long addr,

#define PT_EXITKILL (PTRACE_O_EXITKILL << PT_OPT_FLAG_SHIFT)
#define PT_SUSPEND_SECCOMP (PTRACE_O_SUSPEND_SECCOMP << PT_OPT_FLAG_SHIFT)
+#define PT_ARM64_RAW_REGS (PTRACE_O_ARM64_RAW_REGS << PT_OPT_FLAG_SHIFT)

/* single stepping state bits (used on ARM and PA-RISC) */
#define PT_SINGLESTEP_BIT 31
diff --git a/include/uapi/linux/ptrace.h b/include/uapi/linux/ptrace.h
index 83ee45fa634b..bcc8c362ddd9 100644
--- a/include/uapi/linux/ptrace.h
+++ b/include/uapi/linux/ptrace.h
@@ -7,6 +7,7 @@
/* has the defines to get at the registers. */

#include <linux/types.h>
+#include <asm/ptrace.h>

#define PTRACE_TRACEME 0
#define PTRACE_PEEKTEXT 1
@@ -137,8 +138,14 @@ struct ptrace_syscall_info {
#define PTRACE_O_EXITKILL (1 << 20)
#define PTRACE_O_SUSPEND_SECCOMP (1 << 21)

+/* (1<<28) is reserved for arch specific options. */
+#ifndef _PTRACE_O_ARCH_OPTIONS
+#define _PTRACE_O_ARCH_OPTIONS 0
+#endif
+
#define PTRACE_O_MASK (\
- 0x000000ff | PTRACE_O_EXITKILL | PTRACE_O_SUSPEND_SECCOMP)
+ 0x000000ff | PTRACE_O_EXITKILL | \
+ PTRACE_O_SUSPEND_SECCOMP | _PTRACE_O_ARCH_OPTIONS)

#include <asm/ptrace.h>

--
2.29.2