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f39b4f8899
lld on RISC-V is not yet able to handle undefined weak symbols for non-PIC code in the code model (medany/medium) used by the RISC-V kernel. Both GCC and clang emit an auipc / addi pair of instructions to generate an address relative to the current PC with a 31-bit offset. Undefined weak symbols need to have an address of 0, but the kernel runs with PC values much greater than 2^31, so there is no way to construct a NULL pointer as a PC-relative value. The bfd linker rewrites the instruction pair to use lui / addi with values of 0 to force a NULL pointer address. (There are similar cases for 'ld' becoming auipc / ld that bfd rewrites to lui / ld with an address of 0.) To work around this, compile the kernel with -fPIE when using lld. This does not make the kernel position-independent, but it does force the compiler to indirect address lookups through GOT entries (so auipc / ld against a GOT entry to fetch the address). This adds extra memory indirections for global symbols, so should be disabled once lld is finally fixed. A few 'la' instructions in locore that depend on PC-relative addressing to load physical addresses before paging is enabled have to use auipc / addi and not indirect via GOT entries, so change those to use 'lla' which always uses auipc / addi for both PIC and non-PIC. Submitted by: jrtc27 Sponsored by: DARPA Differential Revision: https://reviews.freebsd.org/D23064 |
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