A function pointer under FDPIC is not a code address. Because each
PT_LOAD segment is placed independently, a pointer has to carry the data
base its callee will need, so it is a two-word descriptor: the entry
point, and the base to install in the PIC register before branching.
R_ARM_FUNCDESC_VALUE says "the thing you are patching is such a
descriptor", and R_ARM_FUNCDESC says "manufacture one and give me its
address".
Both need state a relocation cannot carry. A descriptor's second word is
the *object's* data base, from DT_PLTGOT, and R_ARM_FUNCDESC carves
descriptors from a pool whose cursor has to survive from one relocation
to the next. up_relocate() is handed only a relocation, a resolved
symbol and an address to patch.
arch_data is the existing channel for exactly this -- RISC-V already uses
it to remember a HI20 relocation while its LO12 partner is processed --
but nothing has ever put loader state into it: it is declared zeroed and
written only by up_relocate() itself. So ARCH_ELFDATA_INIT and
ARCH_ELFDATA_FINI are added, seeding the block from the loadinfo before
the relocation loop and reading the cursor back after. Both default to
nothing, so an architecture that does not define them is unaffected, and
RISC-V's use of arch_data is untouched. libelf_relocatedyn() walks both
dynamic tables under one arch_data, so the cursor spans the whole object.
The addend handling is the part that is easy to get wrong. REL format
keeps the addend in place, in the word about to become the entry point,
and a pointer to a static function is referenced through its *section*
symbol -- the value is the section base and the offset, including the
Thumb bit, is entirely in the addend. Dropping it yields an even address
and the core faults trying to execute it as ARM code.
The GOT written into a descriptor is the loading object's own, even for
an imported function, which is what makes a callback work: when the base
firmware's qsort() calls back into a module's comparison function, the
module needs its own data base in the PIC register.
libelf_relocatedyn()'s imported-symbol path needed a change to suit. It
stores the resolved address directly and never calls up_relocate(), which
cannot produce a two-word descriptor, so under FDPIC the resolved value
now goes through up_relocate() and the relocation type decides what to
write.
Implemented for armv7-m and armv8-m, the profiles FDPIC targets; the
other ARM variants gain the arch_data block but no new relocations.
Built and booted mps3-an547:picostest and lm3s6965-ek:qemu-nxflat, the
ELF PIC and NXFLAT users of this code, both unchanged.
Assisted-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>
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