Subject: [PATCH v5 bpf-next 5/7] x86/alternative: introduce text_poke_copy

From: Song Liu <[email-masked]>
Date: Thu, 20 Jan 2022 11:13:03 -0800

This will be used by BPF jit compiler to dump JITed binary to a RX huge
page, and thus allow multiple BPF programs sharing the a huge (2MB) page.

Signed-off-by: Song Liu <[email-masked]>
---
 arch/x86/include/asm/text-patching.h |  1 +
 arch/x86/kernel/alternative.c        | 32 ++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
index b7421780e4e9..4cc18ba1b75e 100644
--- a/arch/x86/include/asm/text-patching.h
+++ b/arch/x86/include/asm/text-patching.h
@@ -44,6 +44,7 @@ extern void text_poke_early(void *addr, const void *opcode, size_t len);
 extern void *text_poke(void *addr, const void *opcode, size_t len);
 extern void text_poke_sync(void);
 extern void *text_poke_kgdb(void *addr, const void *opcode, size_t len);
+extern void *text_poke_copy(void *addr, const void *opcode, size_t len);
 extern int poke_int3_handler(struct pt_regs *regs);
 extern void text_poke_bp(void *addr, const void *opcode, size_t len, const void *emulate);
 
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 23fb4d51a5da..903a415c19fa 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1102,6 +1102,38 @@ void *text_poke_kgdb(void *addr, const void *opcode, size_t len)
 	return __text_poke(addr, opcode, len);
 }
 
+/**
+ * text_poke_copy - Copy instructions into (an unused part of) RX memory
+ * @addr: address to modify
+ * @opcode: source of the copy
+ * @len: length to copy, could be more than 2x PAGE_SIZE
+ *
+ * Not safe against concurrent execution; useful for JITs to dump
+ * new code blocks into unused regions of RX memory. Can be used in
+ * conjunction with synchronize_rcu_tasks() to wait for existing
+ * execution to quiesce after having made sure no existing functions
+ * pointers are live.
+ */
+void *text_poke_copy(void *addr, const void *opcode, size_t len)
+{
+	unsigned long start = (unsigned long)addr;
+	size_t patched = 0;
+
+	if (WARN_ON_ONCE(core_kernel_text(start)))
+		return NULL;
+
+	while (patched < len) {
+		unsigned long ptr = start + patched;
+		size_t s;
+
+		s = min_t(size_t, PAGE_SIZE * 2 - offset_in_page(ptr), len - patched);
+
+		__text_poke((void *)ptr, opcode + patched, s);
+		patched += s;
+	}
+	return addr;
+}
+
 static void do_sync_core(void *info)
 {
 	sync_core();
-- 
2.30.2