As pointed out in #11322 there is a hardware design issue in RISC-V that
affects RV64 relocations. The problem is with how address bits are loaded
into registers via lui / auipc and sign extension.
If the hi20 relocation value happens to have its 32-bit sign bit set, i.e.
value is 0x80000000 (but not negative! i.e. negative in 64-bit format) the
relocation will fail, as the address is erroneously sign extended:
0x00000000_80000000 becomes 0xffffffff_80000000 which is not correct.
Also, make sure the correct opcode is used with PCREL_HI20, it expects
AUIPC (not LUI). The C compiler will never emit such code but when hand-
writing assembly code this can happen.
There is a problem with the current elf loader for risc-v: when a pair of
PCREL_HI20 / LO12 relocations are encountered, it is assumed that these
will follow each other immediately, as follows:
label:
auipc a0, %pcrel_hi(symbol) // R_RISCV_PCREL_HI20
load/store a0, %pcrel_lo(label)(a0) // R_RISCV_PCREL_LO12_I/S
With this assumption, the hi/lo relocations are both done when a hi20
relocation entry is encountered, first to the current instruction (addr)
and to the next instruction (addr + 4).
However, this assumption is wrong. There is nothing in the elf relocation
specification[1] that mandates this. Thus, the hi/lo relocation always
needs to first fixup the hi-part, and when the lo-part is encountered, it
needs to find the corresponding hi relocation entry, via the given "label".
This necessitates (re-)visiting the relocation entries for the current
section as well as looking for "label" in the symbol table.
The NuttX elf loader does not allow such operations to be done in the
machine specific part, so this patch fixes the relocation issue by
introducing an architecture specific cache for the hi20 relocation and
symbol table entries. When a lo12 relocation is encountered, the cache
can be consulted to find the hi20 part.
[1] https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-elf.adoc
since symbols defined in arch/elf.h is also used in other case, for example:
CC: pthread/pthread_testcancel.c machine/arm/gnu_unwind_find_exidx.c:32:8: error: unknown type name '__EIT_entry'
32 | static __EIT_entry *__exidx_start_elf;
| ^~~~~~~~~~~
Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
