[PATCH v2 1/2] powerpc/code-patching: add patch_memory() for writing RO text

[Russell Currey]

powerpc allocates a text poke area of one page that is used by
patch_instruction() to modify read-only text when STRICT_KERNEL_RWX
is enabled.

patch_instruction() is only designed for instructions,
so writing data using the text poke area can only happen 4 bytes
at a time - each with a page map/unmap, pte flush and syncs.

This patch introduces patch_memory(), implementing the same
interface as memcpy(), similar to x86's text_poke() and s390's
s390_kernel_write().  patch_memory() only needs to map the text
poke area once, unless the write would cross a page boundary.

Signed-off-by: Russell Currey <ruscur at russell.cc>
---
v2: Use min_t() instead of min(), fixing the 32-bit build as reported
    by snowpatch.

Some discussion here: https://github.com/linuxppc/issues/issues/375

 arch/powerpc/include/asm/code-patching.h |  1 +
 arch/powerpc/lib/code-patching.c         | 74 ++++++++++++++++++++++++
 2 files changed, 75 insertions(+)

diff --git a/arch/powerpc/include/asm/code-patching.h b/arch/powerpc/include/asm/code-patching.h
index 4ba834599c4d..604211d8380c 100644
--- a/arch/powerpc/include/asm/code-patching.h
+++ b/arch/powerpc/include/asm/code-patching.h
@@ -31,6 +31,7 @@ int create_cond_branch(struct ppc_inst *instr, const u32 *addr,
 int patch_branch(u32 *addr, unsigned long target, int flags);
 int patch_instruction(u32 *addr, struct ppc_inst instr);
 int raw_patch_instruction(u32 *addr, struct ppc_inst instr);
+void *patch_memory(void *dest, const void *src, size_t size);
 
 static inline unsigned long patch_site_addr(s32 *site)
 {
diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
index c5ed98823835..330602aa59f1 100644
--- a/arch/powerpc/lib/code-patching.c
+++ b/arch/powerpc/lib/code-patching.c
@@ -17,6 +17,7 @@
 #include <asm/code-patching.h>
 #include <asm/setup.h>
 #include <asm/inst.h>
+#include <asm/cacheflush.h>
 
 static int __patch_instruction(u32 *exec_addr, struct ppc_inst instr, u32 *patch_addr)
 {
@@ -178,6 +179,74 @@ static int do_patch_instruction(u32 *addr, struct ppc_inst instr)
 
 	return err;
 }
+
+static int do_patch_memory(void *dest, const void *src, size_t size, unsigned long poke_addr)
+{
+	unsigned long patch_addr = poke_addr + offset_in_page(dest);
+
+	if (map_patch_area(dest, poke_addr)) {
+		pr_warn("failed to map %lx\n", poke_addr);
+		return -1;
+	}
+
+	memcpy((u8 *)patch_addr, src, size);
+
+	flush_icache_range(patch_addr, size);
+
+	if (unmap_patch_area(poke_addr)) {
+		pr_warn("failed to unmap %lx\n", poke_addr);
+		return -1;
+	}
+
+	return 0;
+}
+
+/**
+ * patch_memory - write data using the text poke area
+ *
+ * @dest:	destination address
+ * @src:	source address
+ * @size:	size in bytes
+ *
+ * like memcpy(), but using the text poke area. No atomicity guarantees.
+ * Do not use for instructions, use patch_instruction() instead.
+ * Handles crossing page boundaries, though you shouldn't need to.
+ *
+ * Return value:
+ * 	@dest
+ **/
+void *patch_memory(void *dest, const void *src, size_t size)
+{
+	size_t bytes_written, write_size;
+	unsigned long text_poke_addr;
+	unsigned long flags;
+
+	// If the poke area isn't set up, it's early boot and we can just memcpy.
+	if (!this_cpu_read(text_poke_area))
+		return memcpy(dest, src, size);
+
+	local_irq_save(flags);
+	text_poke_addr = (unsigned long)__this_cpu_read(text_poke_area)->addr;
+
+	for (bytes_written = 0;
+	     bytes_written < size;
+	     bytes_written += write_size) {
+		// Write as much as possible without crossing a page boundary.
+		write_size = min_t(size_t,
+				   size - bytes_written,
+				   PAGE_SIZE - offset_in_page(dest + bytes_written));
+
+		if (do_patch_memory(dest + bytes_written,
+				    src + bytes_written,
+				    write_size,
+				    text_poke_addr))
+			break;
+	}
+
+	local_irq_restore(flags);
+
+	return dest;
+}
 #else /* !CONFIG_STRICT_KERNEL_RWX */
 
 static int do_patch_instruction(u32 *addr, struct ppc_inst instr)
@@ -185,6 +254,11 @@ static int do_patch_instruction(u32 *addr, struct ppc_inst instr)
 	return raw_patch_instruction(addr, instr);
 }
 
+void *patch_memory(void *dest, const void *src, size_t size)
+{
+	return memcpy(dest, src, size);
+}
+
 #endif /* CONFIG_STRICT_KERNEL_RWX */
 
 int patch_instruction(u32 *addr, struct ppc_inst instr)
-- 
2.34.1