[PATCH bpf-next v3 1/3] riscv: extend patch_text_nosync() for multiple pages

[Puranjay Mohan]

The patch_insn_write() function currently doesn't work for multiple pages
of instructions, therefore patch_text_nosync() will fail with a page fault
if called with lengths spanning multiple pages.

This commit extends the patch_insn_write() function to support multiple
pages by copying at max 2 pages at a time in a loop. This implementation
is similar to text_poke_copy() function of x86.

Signed-off-by: Puranjay Mohan <puranjay12 at gmail.com>
Reviewed-by: Björn Töpel <bjorn at rivosinc.com>
---
 arch/riscv/kernel/patch.c | 37 ++++++++++++++++++++++++++++++++-----
 1 file changed, 32 insertions(+), 5 deletions(-)

diff --git a/arch/riscv/kernel/patch.c b/arch/riscv/kernel/patch.c
index 575e71d6c8ae..2c97e246f4dc 100644
--- a/arch/riscv/kernel/patch.c
+++ b/arch/riscv/kernel/patch.c
@@ -53,12 +53,18 @@ static void patch_unmap(int fixmap)
 }
 NOKPROBE_SYMBOL(patch_unmap);
 
-static int patch_insn_write(void *addr, const void *insn, size_t len)
+static int __patch_insn_write(void *addr, const void *insn, size_t len)
 {
 	void *waddr = addr;
 	bool across_pages = (((uintptr_t) addr & ~PAGE_MASK) + len) > PAGE_SIZE;
 	int ret;
 
+	/*
+	 * 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
@@ -74,7 +80,7 @@ static int patch_insn_write(void *addr, const void *insn, size_t len)
 		lockdep_assert_held(&text_mutex);
 
 	if (across_pages)
-		patch_map(addr + len, FIX_TEXT_POKE1);
+		patch_map(addr + PAGE_SIZE, FIX_TEXT_POKE1);
 
 	waddr = patch_map(addr, FIX_TEXT_POKE0);
 
@@ -87,15 +93,36 @@ static int patch_insn_write(void *addr, const void *insn, size_t len)
 
 	return ret;
 }
-NOKPROBE_SYMBOL(patch_insn_write);
+NOKPROBE_SYMBOL(__patch_insn_write);
 #else
-static int patch_insn_write(void *addr, const void *insn, size_t len)
+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);
+NOKPROBE_SYMBOL(__patch_insn_write);
 #endif /* CONFIG_MMU */
 
+static int patch_insn_write(void *addr, const void *insn, size_t len)
+{
+	size_t patched = 0;
+	size_t size;
+	int ret = 0;
+
+	/*
+	 * Copy the instructions to the destination address, two pages at a time
+	 * because __patch_insn_write() can only handle 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_write(addr + patched, insn + patched, size);
+
+		patched += size;
+	}
+
+	return ret;
+}
+NOKPROBE_SYMBOL(patch_insn_write);
+
 int patch_text_nosync(void *addr, const void *insns, size_t len)
 {
 	u32 *tp = addr;
-- 
2.40.1