[PATCH v1] GAS: Keep symbols in their own section [PR27566]

[Fangrui Song]

On Thu, Oct 24, 2024 at 10:54 PM Xiao Zeng <zengxiao@eswincomputing.com> wrote:
>
> Hi Jeff and Nelson, an issue has arisen in GCC, described here:
> <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115166>
>
> Afterward, it was thought to be an ld issue, described here:
> <https://sourceware.org/bugzilla/show_bug.cgi?id=27566>
>
> Nelson has done some work on ld, but it hasn’t completely resolved the problem. I understand that this issue may actually stem from as. I have tested the following modification, which can resolve the current issue.
>
> Here is the emails exchange I had with Alan:
> <https://sourceware.org/pipermail/binutils/2024-October/137360.html>
>
> Alan provided some excellent insights.
>
> To help drive the resolution of this issue, could you also offer some advice when you have time?
>
> Thank you both very much.
>
> 2024-10-22 17:43  Xiao Zeng <zengxiao@eswincomputing.com> wrote:
> >
> >PR27566 is categorized under ld:
> ><https://sourceware.org/bugzilla/show_bug.cgi?id=27566>
> >but the root cause of this issue is in gas.
> >
> >The following assembly example is provided:
> >        .section        .rodata
> >        .globl Sym
> >        .type  Sym, @object
> >        .set   Sym, . + 0x1200
> >        .size  Sym, 8
> >
> >readelf reads ELF information:
> >Section Headers:
> >  [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
> >  [ 4] .rodata           PROGBITS        0000000000000000 000040 000000 00   A  0   0  1
> >
> >Symbol table '.symtab' contains 7 entries:
> >   Num:    Value          Size Type    Bind   Vis      Ndx Name
> >     6: 0000000000001200     8 OBJECT  GLOBAL DEFAULT    4 Sym
> >
> >The Sym belongs to the .rodata section, but the size of .rodata is 0.
> >
> >        PR gas/27566
> >       * write.c (size_seg): Update section size.
> >       * testsuite/gas/elf/elf.exp: New test.
> >       * testsuite/gas/elf/pr27566.d: New file.
> >       * testsuite/gas/elf/pr27566.s: Likewise.
> >
> >Signed-off-by: Xiao Zeng <zengxiao@eswincomputing.com>
> >---
> > gas/testsuite/gas/elf/elf.exp   |  1 +
> > gas/testsuite/gas/elf/pr27566.d | 11 +++++++++++
> > gas/testsuite/gas/elf/pr27566.s |  5 +++++
> > gas/write.c                     |  7 +++++++
> > 4 files changed, 24 insertions(+)
> > create mode 100644 gas/testsuite/gas/elf/pr27566.d
> > create mode 100644 gas/testsuite/gas/elf/pr27566.s
> >
> >diff --git a/gas/testsuite/gas/elf/elf.exp b/gas/testsuite/gas/elf/elf.exp
> >index c828c3af25c..6a69acbc109 100644
> >--- a/gas/testsuite/gas/elf/elf.exp
> >+++ b/gas/testsuite/gas/elf/elf.exp
> >@@ -387,4 +387,5 @@ if { [is_elf_format] } then {
> >     run_dump_test "bignums" $dump_opts
> >     run_dump_test "section-symbol-redef"
> >     run_dump_test "pr27228"
> >+    run_dump_test "pr27566"
> > }
> >diff --git a/gas/testsuite/gas/elf/pr27566.d b/gas/testsuite/gas/elf/pr27566.d
> >new file mode 100644
> >index 00000000000..75268cefb20
> >--- /dev/null
> >+++ b/gas/testsuite/gas/elf/pr27566.d
> >@@ -0,0 +1,11 @@
> >+#as:
> >+#name: Keep symbols in their own section (PR 27566)
> >+#readelf: -S --wide
> >+
> >+There are [0-9]+ section headers, starting at offset 0x[0-9a-f]+:
> >+
> >+Section Headers:
> >+ +\[Nr\] Name +Type +Addr(ess|) +Off +Size +ES +Flg +Lk +Inf +Al
> >+#...
> >+ *\[ [1-9]\] *\.rodata +PROGBITS +0*0 +0[0-9a-f]* +0*1208 .*
> >+#pass
> >diff --git a/gas/testsuite/gas/elf/pr27566.s b/gas/testsuite/gas/elf/pr27566.s
> >new file mode 100644
> >index 00000000000..4232df21494
> >--- /dev/null
> >+++ b/gas/testsuite/gas/elf/pr27566.s
> >@@ -0,0 +1,5 @@
> >+        .section        .rodata
> >+        .globl Sym
> >+        .type  Sym, @object
> >+        .set   Sym, . + 0x1200
> >+        .size Sym, 8
> >diff --git a/gas/write.c b/gas/write.c
> >index 18cf18fc830..7f69d7aad44 100644
> >--- a/gas/write.c
> >+++ b/gas/write.c
> >@@ -559,6 +559,7 @@ size_seg (bfd *abfd ATTRIBUTE_UNUSED, asection *sec, void *xxx ATTRIBUTE_UNUSED)
> >   segment_info_type *seginfo;
> >   int x;
> >   valueT size, newsize;
> >+  symbolS *symp;
> >
> >   subseg_change (sec, 0);
> >
> >@@ -577,6 +578,12 @@ size_seg (bfd *abfd ATTRIBUTE_UNUSED, asection *sec, void *xxx ATTRIBUTE_UNUSED)
> >   else
> >     size = 0;
> >
> >+  /* Keep symbols in their own section.  */
> >+  for (symp = symbol_rootP; symp; symp = symbol_next (symp))
> >+    if (S_GET_SEGMENT (symp) == sec
> >+      && size < (S_GET_VALUE (symp) + S_GET_SIZE (symp)))
> >+      size = S_GET_VALUE (symp) + S_GET_SIZE (symp);
> >+
> >   flags = bfd_section_flags (sec);
> >   if (size == 0 && bfd_section_size (sec) != 0 &&
> >     (flags & SEC_HAS_CONTENTS) != 0)
> >--
> >2.17.1
> Thanks
> Xiao Zeng


I commented on the GNU ld bug report
https://sourceware.org/bugzilla/show_bug.cgi?id=27566#c11 .
This looks like a linker relaxation bug in ld riscv. I agree with
Alan's rejection of the assembler patch.