powerpc/ptrace: Expose DEXCR and HDEXCR registers to ptrace

The DEXCR register is of interest when ptracing processes. Currently it
is static, but eventually will be dynamically controllable by a process.
If a process can control its own, then it is useful for it to be
ptrace-able to (e.g., for checkpoint-restore functionality).

It is also relevant to core dumps (the NPHIE aspect in particular),
which use the ptrace mechanism (or is it the other way around?) to
decide what to dump. The HDEXCR is useful here too, as the NPHIE aspect
may be set in the HDEXCR without being set in the DEXCR. Although the
HDEXCR is per-cpu and we don't track it in the task struct (it's useless
in normal operation), it would be difficult to imagine why a hypervisor
would set it to different values within a guest. A hypervisor cannot
safely set NPHIE differently at least, as that would break programs.

Expose a read-only view of the userspace DEXCR and HDEXCR to ptrace.
The HDEXCR is always readonly, and is useful for diagnosing the core
dumps (as the HDEXCR may set NPHIE without the DEXCR setting it).

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Reviewed-by: Russell Currey <ruscur@russell.cc>
[mpe: Use lower_32_bits() rather than open coding]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230616034846.311705-7-bgray@linux.ibm.com

3 files changed, 37 insertions, 1 deletions

diff --git a/arch/powerpc/include/uapi/asm/elf.h b/arch/powerpc/include/uapi/asm/elf.h
index dbc4a5b8d02d..e0d323c808dd 100644
--- a/arch/powerpc/include/uapi/asm/elf.h
+++ b/arch/powerpc/include/uapi/asm/elf.h
@@ -98,6 +98,7 @@
 #define ELF_NEBB	3	/* includes ebbrr, ebbhr, bescr */
 #define ELF_NPMU	5	/* includes siar, sdar, sier, mmcr2, mmcr0 */
 #define ELF_NPKEY	3	/* includes amr, iamr, uamor */
+#define ELF_NDEXCR	2	/* includes dexcr, hdexcr */
 
 typedef unsigned long elf_greg_t64;
 typedef elf_greg_t64 elf_gregset_t64[ELF_NGREG];
diff --git a/arch/powerpc/kernel/ptrace/ptrace-decl.h b/arch/powerpc/kernel/ptrace/ptrace-decl.h
index 463a63eb8cc7..998a84f64804 100644
--- a/arch/powerpc/kernel/ptrace/ptrace-decl.h
+++ b/arch/powerpc/kernel/ptrace/ptrace-decl.h
@@ -57,6 +57,7 @@ enum powerpc_regset {
 	REGSET_TAR,		/* TAR register */
 	REGSET_EBB,		/* EBB registers */
 	REGSET_PMR,		/* Performance Monitor Registers */
+	REGSET_DEXCR,		/* DEXCR registers */
 #endif
 #ifdef CONFIG_PPC_MEM_KEYS
 	REGSET_PKEY,		/* AMR register */
diff --git a/arch/powerpc/kernel/ptrace/ptrace-view.c b/arch/powerpc/kernel/ptrace/ptrace-view.c
index 5fff0d04b23f..f1032fe626f4 100644
--- a/arch/powerpc/kernel/ptrace/ptrace-view.c
+++ b/arch/powerpc/kernel/ptrace/ptrace-view.c
@@ -454,7 +454,36 @@ static int pmu_set(struct task_struct *target, const struct user_regset *regset,
 					 5 * sizeof(unsigned long));
 	return ret;
 }
-#endif
+
+static int dexcr_active(struct task_struct *target, const struct user_regset *regset)
+{
+	if (!cpu_has_feature(CPU_FTR_ARCH_31))
+		return -ENODEV;
+
+	return regset->n;
+}
+
+static int dexcr_get(struct task_struct *target, const struct user_regset *regset,
+		     struct membuf to)
+{
+	if (!cpu_has_feature(CPU_FTR_ARCH_31))
+		return -ENODEV;
+
+	/*
+	 * The DEXCR is currently static across all CPUs, so we don't
+	 * store the target's value anywhere, but the static value
+	 * will also be correct.
+	 */
+	membuf_store(&to, (u64)lower_32_bits(DEXCR_INIT));
+
+	/*
+	 * Technically the HDEXCR is per-cpu, but a hypervisor can't reasonably
+	 * change it between CPUs of the same guest.
+	 */
+	return membuf_store(&to, (u64)lower_32_bits(mfspr(SPRN_HDEXCR_RO)));
+}
+
+#endif /* CONFIG_PPC_BOOK3S_64 */
 
 #ifdef CONFIG_PPC_MEM_KEYS
 static int pkey_active(struct task_struct *target, const struct user_regset *regset)
@@ -615,6 +644,11 @@ static const struct user_regset native_regsets[] = {
 		.size = sizeof(u64), .align = sizeof(u64),
 		.active = pmu_active, .regset_get = pmu_get, .set = pmu_set
 	},
+	[REGSET_DEXCR] = {
+		.core_note_type = NT_PPC_DEXCR, .n = ELF_NDEXCR,
+		.size = sizeof(u64), .align = sizeof(u64),
+		.active = dexcr_active, .regset_get = dexcr_get
+	},
 #endif
 #ifdef CONFIG_PPC_MEM_KEYS
 	[REGSET_PKEY] = {