[PATCH 0/10] ld: Implement DT_RELR for x86

[Fangrui Song]

On 2022-01-07, H.J. Lu via Binutils wrote:
>On Fri, Jan 7, 2022 at 3:46 PM Fangrui Song <maskray@google.com> wrote:
>>
>> On 2022-01-07, H.J. Lu via Libc-alpha wrote:
>> >Hi Nick,
>> >
>> >I'd like to enable DT_RELR for x86 in binutils 2.38.
>> >
>> >
>> >H.J.
>> >---
>>
>> Thanks for the patch!
>>
>> Is the Clang crash
>> https://sourceware.org/pipermail/libc-alpha/2022-January/134992.html
>> fixed now?
>>
>
>It should be fixed.  Please try my current DT_RELR branch.

Thanks. It works.

% rm /tmp/out/custom1/clang
% ninja -C /tmp/out/custom1/clang -v  # find the linker command line
% <linker-command-line> -Wl,--reproduce=/tmp/clang-relassert.tar
% cd /tmp && tar xf clang-relassert.tar && cd clang-relassert
# Remove --chroot from response.txt

# Managed to compile a file with the linked clang 
% ~/Dev/binutils-gdb/out/release/ld/ld-new -z pack-relative-relocs @response.txt -o bfd.relr 
% LD_LIBRARY_PATH=/tmp/glibc/lld/lib:/usr/lib/x86_64-linux-gnu /tmp/glibc/lld/lib/ld-linux-x86-64.so.2 ./bfd.relr -c /tmp/c/a.c -O2 


There are still quite a few even-address R_X86_64_RELATIVE relocations.
Any idea why they are present?


% ~/Dev/binutils-gdb/out/release/binutils/readelf -r bfd.relr
...
Relocation section '.rela.dyn' at offset 0x5fb400 contains 3133 entries:
   Offset          Info           Type           Sym. Value    Sym. Name + Addend
000009902608  000000000008 R_X86_64_RELATIVE                    9281940
000009902610  000000000008 R_X86_64_RELATIVE                    7a4d565
000009902618  000000000008 R_X86_64_RELATIVE                    91acf98
...
Relocation section '.relr.dyn' at offset 0x60f6e0 contains 19910 entries:
   535577 offsets
0000000008f83f58
0000000008f83f60
0000000008f83f68
...

>
>> > 10/10 ld: Add glibc dependency for DT_RELR
>> >
>> > if (!glibc_bfd && startswith (a->vna_nodename, "GLIBC_2."))
>>
>> Thanks for adding the GLIBC_ABI_DT_RELR verneed only if GLIBC_2.* exists, this will make
>> musl/FreeBSD/NetBSD/etc happy.

Does your DT_RELR patch have something like this?

+    // Don't allow the section to shrink, otherwise the size of the section can
+    // oscillate infinitely. Trailing 1s do not decode to more relocations.
+    if (entries_.size() < old_size)
+      entries_.resize(old_size, 1);