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Message-ID: <20251205171446.2814872-16-jremus@linux.ibm.com>
Date: Fri, 5 Dec 2025 18:14:46 +0100
From: Jens Remus <jremus@...ux.ibm.com>
To: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
linux-s390@...r.kernel.org, bpf@...r.kernel.org, x86@...nel.org,
Steven Rostedt <rostedt@...nel.org>
Cc: Jens Remus <jremus@...ux.ibm.com>, Heiko Carstens <hca@...ux.ibm.com>,
Vasily Gorbik <gor@...ux.ibm.com>,
Ilya Leoshkevich <iii@...ux.ibm.com>,
Josh Poimboeuf <jpoimboe@...nel.org>,
Masami Hiramatsu <mhiramat@...nel.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>,
Jiri Olsa <jolsa@...nel.org>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Namhyung Kim <namhyung@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>,
Andrii Nakryiko <andrii@...nel.org>,
Indu Bhagat <indu.bhagat@...cle.com>,
"Jose E. Marchesi" <jemarch@....org>,
Beau Belgrave <beaub@...ux.microsoft.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Florian Weimer <fweimer@...hat.com>, Kees Cook <kees@...nel.org>,
"Carlos O'Donell" <codonell@...hat.com>, Sam James <sam@...too.org>,
Dylan Hatch <dylanbhatch@...gle.com>
Subject: [RFC PATCH v2 15/15] s390/unwind_user/backchain: Enable HAVE_UNWIND_USER_BACKCHAIN
Enable unwinding of user space using back chain on s390. Based on
arch_stack_walk_user_common() in arch/s390/kernel/stacktrace.c.
Note that an invalid RA obtained from the stack frame pointed to by the
back chain is not a valid indication that the IP is still early in the
function prologue and a fallback to the RA and SP register r14 and r15
contents should be made.
Signed-off-by: Jens Remus <jremus@...ux.ibm.com>
---
Notes (jremus):
Changes in RFC v2:
- Adjusted to latest unwind user changes.
- Use struct stack_frame_user and struct stack_frame_vdso_wrapper from
asm/stacktrace.h.
- In topmost frame do not fallback to RA (and SP) register values if
RA is invalid. This is not a valid indication for early prologue.
- In topmost frame use RA and SP register values if they match those
saved in the frame. This indicates early prologue.
arch/s390/Kconfig | 1 +
arch/s390/kernel/Makefile | 2 +
arch/s390/kernel/unwind_user_backchain.c | 112 +++++++++++++++++++++++
3 files changed, 115 insertions(+)
create mode 100644 arch/s390/kernel/unwind_user_backchain.c
diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 52d3f3b3e086..5aeb2abd390f 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -246,6 +246,7 @@ config S390
select HAVE_SETUP_PER_CPU_AREA
select HAVE_SOFTIRQ_ON_OWN_STACK
select HAVE_SYSCALL_TRACEPOINTS
+ select HAVE_UNWIND_USER_BACKCHAIN
select HAVE_UNWIND_USER_SFRAME
select HAVE_VIRT_CPU_ACCOUNTING
select HAVE_VIRT_CPU_ACCOUNTING_IDLE
diff --git a/arch/s390/kernel/Makefile b/arch/s390/kernel/Makefile
index eb06ff888314..eb662e95c5fd 100644
--- a/arch/s390/kernel/Makefile
+++ b/arch/s390/kernel/Makefile
@@ -83,6 +83,8 @@ obj-$(CONFIG_PERF_EVENTS) += perf_pai_crypto.o perf_pai_ext.o
obj-$(CONFIG_TRACEPOINTS) += trace.o
+obj-$(CONFIG_HAVE_UNWIND_USER_BACKCHAIN) += unwind_user_backchain.o
+
# vdso
obj-y += vdso64/
obj-$(CONFIG_COMPAT) += vdso32/
diff --git a/arch/s390/kernel/unwind_user_backchain.c b/arch/s390/kernel/unwind_user_backchain.c
new file mode 100644
index 000000000000..4e10ca43ea36
--- /dev/null
+++ b/arch/s390/kernel/unwind_user_backchain.c
@@ -0,0 +1,112 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#define pr_fmt(fmt) "backchain: " fmt
+
+#include <asm/asm-offsets.h>
+#include <asm/stacktrace.h>
+#include <linux/security.h>
+#include <linux/unwind_user.h>
+#include <linux/unwind_user_backchain.h>
+
+/**
+ * ip_invalid - Perform some basic checks whether an instruction pointer (IP)
+ * taken from an unreliable source is invalid
+ * @ip: The instruction pointer to be validated
+ *
+ * returns whether the instruction pointer is invalid
+ */
+static inline bool ip_invalid(unsigned long ip)
+{
+ /* Architecture requires IP to be 2-byte aligned. */
+ if (ip & 1)
+ return true;
+ if (ip < mmap_min_addr)
+ return true;
+ if (ip >= current->mm->context.asce_limit)
+ return true;
+ return false;
+}
+
+/**
+ * ip_within_vdso - Check whether an instruction pointer (IP) is within vDSO
+ * @ip: The instruction pointer
+ *
+ * returns whether the instruction pointer is within vDSO
+ */
+static inline bool ip_within_vdso(unsigned long ip)
+{
+ return in_range(ip, current->mm->context.vdso_base, vdso_text_size());
+}
+
+/**
+ * arch_unwind_user_next_backchain - Unwind one frame using s390 back chain
+ * @state: The unwind user state
+ *
+ * returns zero when successful, otherwise -EINVAL.
+ */
+int arch_unwind_user_next_backchain(struct unwind_user_state *state)
+{
+ struct stack_frame_user __user *sf;
+ unsigned long sp, ra;
+
+ sf = (void __user *)state->sp;
+
+ /*
+ * In topmost frame check whether IP in early prologue, RA and SP
+ * registers saved, and no new stack frame allocated.
+ */
+ if (state->topmost) {
+ unsigned long ra_reg;
+
+ if (__get_user(ra, (unsigned long __user *)&sf->gprs[8]))
+ return -EINVAL;
+ if (__get_user(sp, (unsigned long __user *)&sf->gprs[9]))
+ return -EINVAL;
+ if (unwind_user_get_ra_reg(&ra_reg))
+ return -EINVAL;
+ if (ra == ra_reg && sp == state->sp)
+ goto done;
+ }
+
+ if (__get_user(sp, (unsigned long __user *)&sf->back_chain))
+ return -EINVAL;
+ if (!sp && ip_within_vdso(state->ip)) {
+ /*
+ * Assume non-standard vDSO user wrapper stack frame.
+ * See vDSO user wrapper code for details.
+ */
+ struct stack_frame_vdso_wrapper *sf_vdso = (void __user *)sf;
+
+ if (__get_user(ra, (unsigned long __user *)&sf_vdso->return_address))
+ return -EINVAL;
+ sf = (void __user *)((unsigned long)sf + STACK_FRAME_VDSO_OVERHEAD);
+ if (__get_user(sp, (unsigned long __user *)&sf->back_chain))
+ return -EINVAL;
+ } else if (!sp) {
+ /* Assume outermost frame reached. */
+ state->done = true;
+ return 0;
+ } else {
+ /*
+ * Assume IP past prologue and new stack frame allocated.
+ * Follow back chain, which then equals the SP at entry.
+ * Skips caller if wrong in topmost frame.
+ */
+ sf = (void __user *)sp;
+ if (__get_user(ra, (unsigned long __user *)&sf->gprs[8]))
+ return -EINVAL;
+ /* Skip validation: ABI requires SP to be saved as well. */
+ }
+
+done:
+ /* Validate SP and RA (ABI requires SP to be 8-byte aligned). */
+ if (sp & 7 || ip_invalid(ra))
+ return -EINVAL;
+
+ state->ip = ra;
+ state->sp = sp;
+ state->fp = 0; /* Cannot unwind FP. */
+ state->topmost = false;
+
+ return 0;
+}
--
2.51.0
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