riscv: implement a memset like function for text

The BPF JIT needs to write invalid instructions to RX regions of memory to
invalidate removed BPF programs. This needs a function like memset() that
can work with RX memory.

Implement patch_text_set_nosync() which is similar to text_poke_set() of
x86.

Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>
Reviewed-by: Pu Lehui <pulehui@huawei.com>
Acked-by: Björn Töpel <bjorn@kernel.org>
Tested-by: Björn Töpel <bjorn@rivosinc.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20230831131229.497941-4-puranjay12@gmail.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: puranjaymohan

arch/riscv/include/asm/patch.h

@@ -7,6 +7,7 @@
 #define _ASM_RISCV_PATCH_H
 
 int patch_text_nosync(void *addr, const void *insns, size_t len);
+int patch_text_set_nosync(void *addr, u8 c, size_t len);
 int patch_text(void *addr, u32 *insns, int ninsns);
 
 extern int riscv_patch_in_stop_machine;

arch/riscv/kernel/patch.c

@@ -6,6 +6,7 @@
 #include <linux/spinlock.h>
 #include <linux/mm.h>
 #include <linux/memory.h>
+#include <linux/string.h>
 #include <linux/uaccess.h>
 #include <linux/stop_machine.h>
 #include <asm/kprobes.h>
@@ -53,6 +54,39 @@ static void patch_unmap(int fixmap)
 }
 NOKPROBE_SYMBOL(patch_unmap);
 
+static int __patch_insn_set(void *addr, u8 c, size_t len)
+{
+	void *waddr = addr;
+	bool across_pages = (((uintptr_t)addr & ~PAGE_MASK) + len) > PAGE_SIZE;
+
+	/*
+	 * Only two pages can be mapped at a time for writing.
+	 */
+	if (len + offset_in_page(addr) > 2 * PAGE_SIZE)
+		return -EINVAL;
+	/*
+	 * Before reaching here, it was expected to lock the text_mutex
+	 * already, so we don't need to give another lock here and could
+	 * ensure that it was safe between each cores.
+	 */
+	lockdep_assert_held(&text_mutex);
+
+	if (across_pages)
+		patch_map(addr + PAGE_SIZE, FIX_TEXT_POKE1);
+
+	waddr = patch_map(addr, FIX_TEXT_POKE0);
+
+	memset(waddr, c, len);
+
+	patch_unmap(FIX_TEXT_POKE0);
+
+	if (across_pages)
+		patch_unmap(FIX_TEXT_POKE1);
+
+	return 0;
+}
+NOKPROBE_SYMBOL(__patch_insn_set);
+
 static int __patch_insn_write(void *addr, const void *insn, size_t len)
 {
 	void *waddr = addr;
@@ -95,13 +129,56 @@ static int __patch_insn_write(void *addr, const void *insn, size_t len)
 }
 NOKPROBE_SYMBOL(__patch_insn_write);
 #else
+static int __patch_insn_set(void *addr, u8 c, size_t len)
+{
+	memset(addr, c, len);
+
+	return 0;
+}
+NOKPROBE_SYMBOL(__patch_insn_set);
+
 static int __patch_insn_write(void *addr, const void *insn, size_t len)
 {
 	return copy_to_kernel_nofault(addr, insn, len);
 }
 NOKPROBE_SYMBOL(__patch_insn_write);
 #endif /* CONFIG_MMU */
 
+static int patch_insn_set(void *addr, u8 c, size_t len)
+{
+	size_t patched = 0;
+	size_t size;
+	int ret = 0;
+
+	/*
+	 * __patch_insn_set() can only work on 2 pages at a time so call it in a
+	 * loop with len <= 2 * PAGE_SIZE.
+	 */
+	while (patched < len && !ret) {
+		size = min_t(size_t, PAGE_SIZE * 2 - offset_in_page(addr + patched), len - patched);
+		ret = __patch_insn_set(addr + patched, c, size);
+
+		patched += size;
+	}
+
+	return ret;
+}
+NOKPROBE_SYMBOL(patch_insn_set);
+
+int patch_text_set_nosync(void *addr, u8 c, size_t len)
+{
+	u32 *tp = addr;
+	int ret;
+
+	ret = patch_insn_set(tp, c, len);
+
+	if (!ret)
+		flush_icache_range((uintptr_t)tp, (uintptr_t)tp + len);
+
+	return ret;
+}
+NOKPROBE_SYMBOL(patch_text_set_nosync);
+
 static int patch_insn_write(void *addr, const void *insn, size_t len)
 {
 	size_t patched = 0;