[PATCH v4 2/2] arch/riscv: Enable kprobes when CONFIG_MODULES=n

[Jarkko Sakkinen <jarkko@kernel.org>]

Tacing with kprobes while running a monolithic kernel is currently
impossible due the kernel module allocator dependency.

Address the issue by implementing textmem API for RISC-V.

Link: https://www.sochub.fi # for power on testing new SoC's with a minimal stack
Link: https://lore.kernel.org/all/20220608000014.3054333-1-jarkko@profian.com/ # continuation
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
---
v4:
- Include linux/execmem.h.
v3:
- Architecture independent parts have been split to separate patches.
- Do not change arch/riscv/kernel/module.c as it is out of scope for
  this patch set now.
v2:
- Better late than never right? :-)
- Focus only to RISC-V for now to make the patch more digestable. This
  is the arch where I use the patch on a daily basis to help with QA.
- Introduce HAVE_KPROBES_ALLOC flag to help with more gradual migration.
---
 arch/riscv/Kconfig          |  1 +
 arch/riscv/kernel/Makefile  |  3 +++
 arch/riscv/kernel/execmem.c | 22 ++++++++++++++++++++++
 kernel/kprobes.c            |  2 +-
 4 files changed, 27 insertions(+), 1 deletion(-)
 create mode 100644 arch/riscv/kernel/execmem.c

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index e3142ce531a0..499512fb17ff 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -132,6 +132,7 @@ config RISCV
 	select HAVE_KPROBES if !XIP_KERNEL
 	select HAVE_KPROBES_ON_FTRACE if !XIP_KERNEL
 	select HAVE_KRETPROBES if !XIP_KERNEL
+	select HAVE_ALLOC_EXECMEM if !XIP_KERNEL
 	# https://github.com/ClangBuiltLinux/linux/issues/1881
 	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if !LD_IS_LLD
 	select HAVE_MOVE_PMD
diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
index 604d6bf7e476..337797f10d3e 100644
--- a/arch/riscv/kernel/Makefile
+++ b/arch/riscv/kernel/Makefile
@@ -73,6 +73,9 @@ obj-$(CONFIG_SMP)		+= cpu_ops.o
 
 obj-$(CONFIG_RISCV_BOOT_SPINWAIT) += cpu_ops_spinwait.o
 obj-$(CONFIG_MODULES)		+= module.o
+ifeq ($(CONFIG_ALLOC_EXECMEM),y)
+obj-y				+= execmem.o
+endif
 obj-$(CONFIG_MODULE_SECTIONS)	+= module-sections.o
 
 obj-$(CONFIG_CPU_PM)		+= suspend_entry.o suspend.o
diff --git a/arch/riscv/kernel/execmem.c b/arch/riscv/kernel/execmem.c
new file mode 100644
index 000000000000..3e52522ead32
--- /dev/null
+++ b/arch/riscv/kernel/execmem.c
@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include <linux/mm.h>
+#include <linux/execmem.h>
+#include <linux/vmalloc.h>
+#include <asm/sections.h>
+
+void *alloc_execmem(unsigned long size, gfp_t /* gfp */)
+{
+	return __vmalloc_node_range(size, 1, MODULES_VADDR,
+				    MODULES_END, GFP_KERNEL,
+				    PAGE_KERNEL, 0, NUMA_NO_NODE,
+				    __builtin_return_address(0));
+}
+
+void free_execmem(void *region)
+{
+	if (in_interrupt())
+		pr_warn("In interrupt context: vmalloc may not work.\n");
+
+	vfree(region);
+}
diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index a1a547723c3c..87fd8c14a938 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -119,7 +119,7 @@ void __weak *alloc_insn_page(void)
 	 * for most of the architectures.
 	 * (e.g. x86-64 needs this to handle the %rip-relative fixups.)
 	 */
-	return alloc_execmem(PAGE_SIZE);
+	return alloc_execmem(PAGE_SIZE, GFP_KERNEL);
 }
 
 static void free_insn_page(void *page)
-- 
2.44.0