[PATCH] ARM: Thumb-2: Relax relocation requirements for non-function symbols

[Dave Martin]

The "Thumb bit" of a symbol is only really meaningful for function
symbols (STT_FUNC).

However, sometimes a branch is relocated against a non-function
symbol; for example, PC-relative branches to anonymous assembler
local symbols are typically fixed up against the start-of-section
symbol, which is not a function symbol.  Some inline assembler
generates references of this type, such as fixup code generated by
macros in <asm/uaccess.h>.

The existing relocation code for R_ARM_THM_CALL/R_ARM_THM_JUMP24
interprets this case as an error, because the target symbol appears
to be an ARM symbol; but this is really not the case, since the
target symbol is just a base in these cases.  The addend defines
the precise offset to the target location, but since the addend is
encoded in a non-interworking Thumb branch instruction, there is no
explicit Thumb bit in the addend.  Because these instructions never
interwork, the implied Thumb bit in the addend is 1, and the
destination is Thumb by definition.

This patch removes the extraneous Thumb bit check for non-function
symbols, enabling modules containing the affected relocation types
to be loaded.  No modification to the actual relocation code is
required, since this code does not take bit[0] of the
location->destination offset into account in any case.

Function symbols are always checked for interworking conflicts, as
before.

Signed-off-by: Dave Martin <dave.martin at linaro.org>
---
 arch/arm/kernel/module.c |   13 +++++++++++--
 1 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/arm/kernel/module.c b/arch/arm/kernel/module.c
index fee7c36..016d6a0 100644
--- a/arch/arm/kernel/module.c
+++ b/arch/arm/kernel/module.c
@@ -193,8 +193,17 @@ apply_relocate(Elf32_Shdr *sechdrs, const char *strtab, unsigned int symindex,
 				offset -= 0x02000000;
 			offset += sym->st_value - loc;
 
-			/* only Thumb addresses allowed (no interworking) */
-			if (!(offset & 1) ||
+			/*
+			 * For function symbols, only Thumb addresses are
+			 * allowed (no interworking).
+			 *
+			 * For non-function symbols, the destination
+			 * has no specific ARM/Thumb disposition, so
+			 * the branch is resolved under the assumption
+			 * that interworking is not required.
+			 */
+			if ((ELF32_ST_TYPE(sym->st_info) == STT_FUNC &&
+				!(offset & 1)) ||
 			    offset <= (s32)0xff000000 ||
 			    offset >= (s32)0x01000000) {
 				pr_err("%s: section %u reloc %u sym '%s': relocation %u out of range (%#lx -> %#x)\n",
-- 
1.7.4.1