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

Fangrui Song i@maskray.me
Thu Mar 5 06:41:00 GMT 2020 
For convenience, I will use some notations:

max_input_align: maximum of input section alignments.
addr_tree: output section address

On 2020-03-04, Alan Modra wrote:
>On Tue, Mar 03, 2020 at 10:39:45PM -0800, Fangrui Song wrote:
>> The implementation is complex. For users to understand, I think it
>> will be helpful to have something more detailed in
>> https://sourceware.org/binutils/docs/ld/Output-Section-Address.html#Output-Section-Address
>>
>> If my understanding is correct
>> https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=233bf4f847b136705247e2f7f11bae41c72448a4
>>  makes the output section address override sh_addralign computed from
>> the maximum of input section alignments.
>
>Right.
>
>> So, generally the rules are:
>> * The max of ALIGN and (the maximum of input section alignments) is taken.
>> * The output section address overrides the above. If sh_addr %
>> alignment != 0, set sh_addralign to the largest alignment that makes
>> sh_addr%alignment=0
>>   In this case, should the linker emit a warning?
>
>I don't think so.  The input sections are still aligned within the
>output section to their required alignment.
>
>> * ALIGN and the output section address cannot be specified at the same
>> time. This is considered a linker script "undefined behavior". Users
>> should not rely on a particular result.
>
>I'm not going to make that change for ld.bfd.  I said it probably
>would have been better if ALIGN for output section statements hadn't
>been invented, but once there are users for a script feature it can't
>be removed without a good reason.

I take ALIGN as a way to overalign an output section.
When ALIGN < max_input_align, do we agree that sh_addralign = max(ALIGN, max_input_align) = max_input_align ?

When both addr_tree and ALIGN are specified (what I called "undefined behavior"), and addr_tree is misaligned,
sh_addralign can be decreased from max(ALIGN,max_input_align) to
(addr_tree|max(ALIGN,max_input_align)) & -(addr_tree|max(ALIGN,max_input_align))

Commit 233bf4f847b136705247e2f7f11bae41c72448a4 is made so that
"The value of sh_addr must be congruent to 0, modulo the value of sh_addralign."
is obeyed.

Another view is that the user intentionally breaks the ELF rule. We can keep
sh_addralign as max(ALIGN,max_input_align) and emit a warning along the line of:

   warning: address (0x10010) of section .foo is not a multiple of alignment (32)

>> --warn-section-align may be out of place. It can be noisy for normal
>> output section descriptions like    .foo : ALIGN(16) { ... }  without
>> a preceding dot advancing to a multiple of 16.

   /* Without this assignment, the ALIGN(16) below will likely report a warning */
   . = ALIGN(16);
   
   .foo : ALIGN(16) { ... } 

Does this suggest that --warn-section-align is not very useful?
Keep reading.

>It's even more noisy when relaxation is enabled..

https://sourceware.org/ml/binutils/2020-03/msg00107.html does not fix
the --warn-section-align version of PR25570.

# My original example.
cat > a.s <<e
   .globl _start; _start: ret
   .section .data.rel.ro,"aw"; .balign 8; .byte 0
   .data; .byte 0
   .section .data2,"aw"; .balign 8; .byte 0
   .bss; .balign 32; .byte 0
e
as a.s -o a.o

% ./ld-new a.o -o a --warn-section-align            
./ld-new: warning: start of section .got changed by 7
./ld-new: warning: start of section .got.plt changed by 7
./ld-new: warning: start of section .data2 changed by 6
./ld-new: warning: start of section .bss changed by 23
./ld-new: warning: start of section .data.rel.ro changed by 4088
./ld-new: warning: start of section .got changed by 4088
./ld-new: warning: start of section .got.plt changed by 4088
./ld-new: warning: start of section .data2 changed by 4096
./ld-new: warning: start of section .bss changed by 4096
./ld-new: warning: start of section .rela.dyn changed by 56
./ld-new: warning: start of section .rela.plt changed by 56
./ld-new: warning: start of section .data.rel.ro changed by -4088
./ld-new: warning: start of section .got changed by -4088
./ld-new: warning: start of section .got.plt changed by -4088
./ld-new: warning: start of section .data2 changed by -4096
./ld-new: warning: start of section .bss changed by -4096
./ld-new: warning: start of section .data.rel.ro changed by 4088
./ld-new: warning: start of section .got changed by 4088
./ld-new: warning: start of section .got.plt changed by 4088
./ld-new: warning: start of section .data2 changed by 4096
./ld-new: warning: start of section .bss changed by 4096

This also demonstrates how annoying --warn-section-align can be.
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