[PATCH v2 4/4] MIPS: Support R_MIPS_PC{16,21,26} rel-style relocs

[Paul Burton]

MIPS32 code uses rel-style relocs, and MIPS32r6 modules may include
R_MIPS_PC16, R_MIPS_PC21 & R_MIPS_PC26 relocations. We thus need to
support these relocations in order to load MIPS32r6 kernel modules. This
patch adds such support, which is similar to the rela-style support in
module-rela.c but making use of the implicit addend from the instruction
encoding.

Signed-off-by: Paul Burton <paul.burton@xxxxxxxxxx>

---

Changes in v2:
- Unify PC{16,21,26} implementation.

 arch/mips/kernel/module.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 50 insertions(+), 1 deletion(-)

diff --git a/arch/mips/kernel/module.c b/arch/mips/kernel/module.c
index e16624f..ff5d97d 100644
--- a/arch/mips/kernel/module.c
+++ b/arch/mips/kernel/module.c
@@ -182,13 +182,62 @@ out_danger:
 	return -ENOEXEC;
 }
 
+static int apply_r_mips_pc_rel(struct module *me, u32 *location, Elf_Addr v,
+			       unsigned bits)
+{
+	unsigned long mask = GENMASK(bits - 1, 0);
+	unsigned long se_bits;
+	long offset;
+
+	if (v % 4) {
+		pr_err("module %s: dangerous R_MIPS_PC%u REL relocation\n",
+		       me->name, bits);
+		return -ENOEXEC;
+	}
+
+	/* retrieve & sign extend implicit addend */
+	offset = *location & mask;
+	offset |= (offset & BIT(bits - 1)) ? ~mask : 0;
+
+	offset += ((long)v - (long)location) >> 2;
+
+	/* check the sign bit onwards are identical - ie. we didn't overflow */
+	se_bits = (offset & BIT(bits - 1)) ? ~0ul : 0;
+	if ((offset & ~mask) != (se_bits & ~mask)) {
+		pr_err("module %s: relocation overflow\n", me->name);
+		return -ENOEXEC;
+	}
+
+	*location = (*location & ~mask) | (offset & mask);
+
+	return 0;
+}
+
+static int apply_r_mips_pc16_rel(struct module *me, u32 *location, Elf_Addr v)
+{
+	return apply_r_mips_pc_rel(me, location, v, 16);
+}
+
+static int apply_r_mips_pc21_rel(struct module *me, u32 *location, Elf_Addr v)
+{
+	return apply_r_mips_pc_rel(me, location, v, 21);
+}
+
+static int apply_r_mips_pc26_rel(struct module *me, u32 *location, Elf_Addr v)
+{
+	return apply_r_mips_pc_rel(me, location, v, 26);
+}
+
 static int (*reloc_handlers_rel[]) (struct module *me, u32 *location,
 				Elf_Addr v) = {
 	[R_MIPS_NONE]		= apply_r_mips_none,
 	[R_MIPS_32]		= apply_r_mips_32_rel,
 	[R_MIPS_26]		= apply_r_mips_26_rel,
 	[R_MIPS_HI16]		= apply_r_mips_hi16_rel,
-	[R_MIPS_LO16]		= apply_r_mips_lo16_rel
+	[R_MIPS_LO16]		= apply_r_mips_lo16_rel,
+	[R_MIPS_PC16]		= apply_r_mips_pc16_rel,
+	[R_MIPS_PC21_S2]	= apply_r_mips_pc21_rel,
+	[R_MIPS_PC26_S2]	= apply_r_mips_pc26_rel,
 };
 
 int apply_relocate(Elf_Shdr *sechdrs, const char *strtab,
-- 
2.7.0