[PATCH v2 3/7] riscv: Fix auipc+jalr relocation range checks

[Emil Renner Berthing]

RISC-V can do PC-relative jumps with a 32bit range using the following
two instructions:

	auipc	t0, imm20	; t0 = PC + imm20 * 2^12
	jalr	ra, t0, imm12	; ra = PC + 4, PC = t0 + imm12,

Crucially both the 20bit immediate imm20 and the 12bit immediate imm12
are treated as two's-complement signed values. For this reason the
immediates are usually calculated like this:

	imm20 = (offset + 0x800) >> 12
	imm12 = offset & 0xfff

..where offset is the signed offset from the auipc instruction. When
the 11th bit of offset is 0 the addition of 0x800 doesn't change the top
20 bits and imm12 considered positive. When the 11th bit is 1 the carry
of the addition by 0x800 means imm20 is one higher, but since imm12 is
then considered negative the two's complement representation means it
all cancels out nicely.

However, this addition by 0x800 (2^11) means an offset greater than or
equal to 2^31 - 2^11 would overflow so imm20 is considered negative and
result in a backwards jump. Similarly the lower range of offset is also
moved down by 2^11 and hence the true 32bit range is

	[-2^31 - 2^11, 2^31 - 2^11)

Signed-off-by: Emil Renner Berthing <kernel at esmil.dk>
---
 arch/riscv/kernel/module.c | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/arch/riscv/kernel/module.c b/arch/riscv/kernel/module.c
index 3d33442226e7..a75ccf3a6ce8 100644
--- a/arch/riscv/kernel/module.c
+++ b/arch/riscv/kernel/module.c
@@ -13,6 +13,18 @@
 #include <linux/pgtable.h>
 #include <asm/sections.h>
 
+static inline bool riscv_insn_valid_32bit_offset(ptrdiff_t val)
+{
+	if (IS_ENABLED(CONFIG_32BIT))
+		return true;
+
+	/*
+	 * auipc+jalr can reach any PC-relative offset in the range
+	 * [-2^31 - 2^11, 2^31 - 2^11)
+	 */
+	return (-(1L << 31) - (1L << 11)) <= val && val < ((1L << 31) - (1L << 11));
+}
+
 static int riscv_insn_rmw(void *location, u32 keep, u32 set)
 {
 	u16 *parcel = location;
@@ -111,7 +123,7 @@ static int apply_r_riscv_pcrel_hi20_rela(struct module *me, void *location,
 {
 	ptrdiff_t offset = (void *)v - location;
 
-	if (offset != (s32)offset) {
+	if (!riscv_insn_valid_32bit_offset(offset)) {
 		pr_err(
 		  "%s: target %016llx can not be addressed by the 32-bit offset from PC = %p\n",
 		  me->name, (long long)v, location);
@@ -201,10 +213,9 @@ static int apply_r_riscv_call_plt_rela(struct module *me, void *location,
 				       Elf_Addr v)
 {
 	ptrdiff_t offset = (void *)v - location;
-	s32 fill_v = offset;
 	u32 hi20, lo12;
 
-	if (offset != fill_v) {
+	if (!riscv_insn_valid_32bit_offset(offset)) {
 		/* Only emit the plt entry if offset over 32-bit range */
 		if (IS_ENABLED(CONFIG_MODULE_SECTIONS)) {
 			offset = (void *)module_emit_plt_entry(me, v) - location;
@@ -226,10 +237,9 @@ static int apply_r_riscv_call_rela(struct module *me, void *location,
 				   Elf_Addr v)
 {
 	ptrdiff_t offset = (void *)v - location;
-	s32 fill_v = offset;
 	u32 hi20, lo12;
 
-	if (offset != fill_v) {
+	if (!riscv_insn_valid_32bit_offset(offset)) {
 		pr_err(
 		  "%s: target %016llx can not be addressed by the 32-bit offset from PC = %p\n",
 		  me->name, (long long)v, location);
-- 
2.35.1