On Wed, Jul 21, 2021 at 1:49 PM Jessica Clarke [off-list ref] wrote:

On 11 Jul 2021, at 14:53, Bin Meng [off-list ref] wrote:

quoted

On Sat, Jul 10, 2021 at 9:23 PM Bin Meng [off-list ref] wrote:

quoted

On Sat, Jul 10, 2021 at 3:35 AM Jessica Clarke [off-list ref] wrote:

quoted

This is intended to mirror the Linux kernel. Building with CC=clang will
use Clang as the compiler but default to using the existing binutils.
Building with LLVM=1 will default to using Clang and LLVM binutils.

Whilst GCC will accept the -N linker option and forward it on to the
linker, Clang will not, and so in order to support both compilers we
must use -Wl, to forward it to the linker as is required for most other
linker options.

Signed-off-by: Jessica Clarke <redacted>
---
Makefile  | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++-----
README.md | 43 ++++++++++++++++++++++++++++++++++--
2 files changed, 100 insertions(+), 8 deletions(-)

Here are test results:

Building with "riscv64-linux-gcc",

$ file build/platform/generic/firmware/fw_dynamic.elf
build/platform/generic/firmware/fw_dynamic.elf: ELF 64-bit LSB
executable, UCB RISC-V, version 1 (SYSV), dynamically linked, with
debug_info, not stripped

$ riscv64-linux-readelf -r build/platform/generic/firmware/fw_dynamic.elf
Relocation section '.rela.dyn' at offset 0x140f8 contains 184 entries:
 Offset          Info           Type           Sym. Value    Sym. Name + Addend
000080013090  000000000003 R_RISCV_RELATIVE                     80000db4
...
000080013628  000200000002 R_RISCV_64        0000000080013720
fdt_serial_uart8250 + 0
000080013830  000d00000002 R_RISCV_64        00000000800138d8
fdt_reset_sifive_test + 0

Building with "LLVM=1",

$ file build/platform/generic/firmware/fw_dynamic.elf
build/platform/generic/firmware/fw_dynamic.elf: ELF 64-bit LSB shared
object, UCB RISC-V, version 1 (SYSV), dynamically linked, with
debug_info, not stripped

$ riscv64-linux-readelf -r build/platform/generic/firmware/fw_dynamic.elf
Relocation section '.rela.dyn' at offset 0x17d98 contains 188 entries:
 Offset          Info           Type           Sym. Value    Sym. Name + Addend
000080017000  000000000003 R_RISCV_RELATIVE                     8000b680
000080017030  000000000003 R_RISCV_RELATIVE                     8001b1b8
...
000080017c90  000000000003 R_RISCV_RELATIVE                     80017628

There are two differences:

1. LLVM toolchain generates a "shared object" firmware image, while
GCC generates "executable".
2. LLVM one has 4 more entries in .rela.dyn than the GCC. All entries
of LLVM have the R_RISCV_RELATIVE type, but GCC one has two R_RISCV_64
entries.

Do you have any explanations on these 2 differences? Are these
possible toolchain bugs?

[Hm, I composed this on the 11th but seems I never sent it...]

The first one smells to me like GNU ld is wrong, as executables are
inherently not position-independent, PIEs are always shared objects,
and I?m pretty sure that?s true of the recent -static-pie support too.
In practice for our use cases it doesn?t matter though. It seems this
is yet another undocumented, likely unintended (as it?s a very old
legacy, and mostly unused, option) consequence of using -N/--omagic (in
that it just blindly sets various flags internally and nobody thought
about whether that made sense once -pie was added). Incidentally, I
don?t think we actually need -N/--omagic any more, but that?s a
separate thing.

For the minor difference in number of relocations, that probably just
comes down to minor codegen differences and I wouldn?t worry about it;
with a large enough code base small differences are to be expected.

As for R_RISCV_64, there?s no reason for GNU ld to emit R_RISCV_64
here. It?s technically correct but entirely unnecessary (and can break
legitimate code that assumes only R_RISCV_RELATIVE gets emitted, which

Fortunately OpenSBI, as well as U-Boot, handle both R_RISCV_RELATIVE
and R_RISCV_{64,32} here, so they are not broken due to these
unnecessary entries.

*should* be the case; kernel and run-time linker self-relocation code,
that looks a lot like what OpenSBI is doing here, often likes to assume
that, possibly with R_RISCV_IRELATIVE too if IFUNCs are used). I see it
locally for fdt_serial_uart8250 and fdt_reset_sifive which should in no
way be special, there?s nothing stopping those being evaluated at link
time and leaving R_RISCV_RELATIVE to adjust them at run time as needed
like with all the other symbols.

So I?d regard -N + -pie giving EXEC not DYN, and R_RISCV_64 being
emitted here, as being GNU ld sort-of-bugs, albeit with the former
being extremely ill-defined over what that combination means (beyond
?it does what it does?), with LLD?s output for both being what I would
expect.

Thanks a lot for the explanation! Someone can file a defect to the GNU ld :)

Regards,
Bin