Re: Alignment of .text sections in object files

[LIU Hao <lh_mouse@126.com>]

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在 2022-05-23 23:21, Richard Campbell 写道:
> On Mon, May 23, 2022 at 8:03 AM LIU Hao <lh_mouse@126.com> wrote:
>>
>> So my question is, is
>> it really necessary for .text sections in object files to be aligned to 16-byte boundaries? And
>> could you please kindly elaborate it a little?
> 
> I believe the relevant requirement is found in the "Optional Header
> Windows-Specific Fields (Image Only)" part of the
> https://docs.microsoft.com/en-us/windows/win32/debug/pe-format
> specification.
> 
> This header "contain[s] additional information that is required by the
> linker and loader in Windows," and in particular has a
> "SectionAlignment" field:  "The alignment (in bytes) of sections when
> they are loaded into memory. It must be greater than or equal to
> FileAlignment. The default is the page size for the architecture."
> 
> Turning to FileAlignment:  "The alignment factor (in bytes) that is
> used to align the raw data of sections in the image file. The value
> should be a power of 2 between 512 and 64 K, inclusive. The default is
> 512. If the SectionAlignment is less than the architecture's page
> size, then FileAlignment must match SectionAlignment."
> 
> ...I'd expect you to see the alignment set to 0x200, which is the
> default and the minimum for FileAlignment, so a 16 byte boundary seems
> odd to me.  There's a little more information here:
> https://docs.microsoft.com/en-us/archive/msdn-magazine/2002/february/inside-windows-win32-portable-executable-file-format-in-detail
> 
> Best regards,
> 
> -Richard Campbell

[please keep CC'ing me as I am not subscribed to this list]


The FileAlignment is a totally different thing. Typically the linker (GNU LD for example) is 
responsible for combining all .text sections from object files to create a final .text section for 
the executable output, whose offset (in the output file) is specified by FileAlignment, which cannot 
be below 512.

However we were talking about something before linking: the alignment of .text sections in 
individual object files. They have a default alignment of 16 bytes, shown in objdump as `2**4`.




-- 
Best regards,
LIU Hao

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