Re: [ld] section address : ALIGN(align) and the maximum of input section alignments

[Alan Modra <amodra@gmail.com>]

On Tue, Mar 03, 2020 at 03:58:12PM -0800, Fangrui Song wrote:
> Thanks. For the example below, do you agree with the comments in a.x below?
> 
> % cat a.s
> .globl _start; _start: ret
> .section .data.rel.ro,"aw"; .balign 8; .byte 0
> .data; .byte 0
> .section .data2,"aw"; .balign 8; .byte 0
> .section .data3,"aw"; .balign 32; .byte 0
> .bss; .balign 32; .byte 0
> % as a.s -o a.o
> 
> % cat a.x
> SECTIONS {
>   .text 0x10000 : { *(.text) }
>   /* sh_addr is 0x10010. Specifying both address and ALIGN should be
> disallowed. */
>   .data.rel.ro . : ALIGN(16) { *(.data.rel.ro) }
> 
>   .data 0x20000 : { *(.data) }
>   /* sh_addr is 0x20001. Should there be a warning that sh_addralign
> is 8? Even --warn-section-align does not warn. */

That is a bug.

The ELF gABI says in part of sh_addralign:  "The value of sh_addr must
be congruent to 0, modulo the value of sh_addralign."

	* elf.c (elf_fake_sections): Ensure sh_addralign is such that
	sh_addr mod sh_addalign is zero.

diff --git a/bfd/elf.c b/bfd/elf.c
index fcd84d2d17..c4d6718aaa 100644
--- a/bfd/elf.c
+++ b/bfd/elf.c
@@ -3192,6 +3192,7 @@ elf_fake_sections (bfd *abfd, asection *asect, void *fsarg)
   unsigned int sh_type;
   const char *name = asect->name;
   bfd_boolean delay_st_name_p = FALSE;
+  bfd_vma mask;
 
   if (arg->failed)
     {
@@ -3291,7 +3292,10 @@ elf_fake_sections (bfd *abfd, asection *asect, void *fsarg)
       arg->failed = TRUE;
       return;
     }
-  this_hdr->sh_addralign = (bfd_vma) 1 << asect->alignment_power;
+  /* Set sh_addralign to the highest power of two given by alignment
+     consistent with the section VMA.  Linker scripts can force VMA.  */
+  mask = ((bfd_vma) 1 << asect->alignment_power) | this_hdr->sh_addr;
+  this_hdr->sh_addralign = mask & -mask;
   /* The sh_entsize and sh_info fields may have been set already by
      copy_private_section_data.  */
 

>   .data2 . : { *(.data2) }
>   /* sh_addr is 0x20020. The input section alignment wins. */
>   .data3 : ALIGN(16) { *(.data3) }
>   /* sh_addr is 0x20030. Specifying both address and ALIGN should be
> disallowed. */
>   .bss . : ALIGN(16) { *(.bss) }
> }
> 
> % ld.bfd -T a.x a.s -o a
> 
> If specifying both Output Section Address and ALIGN is disallowed,
> there should be no "changing start of section" warning when
> --warn-section-align is not specified. Is my understanding correct?
> 
> BTW, I filed a bug about duplicate "changing start of section"
> warnings https://sourceware.org/bugzilla/show_bug.cgi?id=25570

-- 
Alan Modra
Australia Development Lab, IBM