[PATCH] arm64/debug: don't duplicate filenames.

[jamie.iles@oracle.com (Jamie Iles)]

On Thu, Nov 09, 2017 at 02:31:22PM +0000, Dave Martin wrote:
> On Thu, Nov 09, 2017 at 01:08:51PM +0000, Jamie Iles wrote:
> > Hi Dave,
> > 
> > On Thu, Nov 09, 2017 at 12:35:04PM +0000, Dave Martin wrote:
> > > On Thu, Nov 09, 2017 at 12:14:40PM +0000, Jamie Iles wrote:
> > > > Rather than explicitly pushing the filename into .rodata.str, use a
> > > > compiler generated string literal and use the address of that as an
> > > > input constraint to the inline assembly.  This allows the compiler to
> > > > emit only one version of the string without relying on the linker to
> > > > deduplicate.
> > > 
> > > But if the linker does deduplicate, why does it matter?
> > 
> > It's not broken, but at the moment we have multiple copies in the object 
> > file then we're relying on the linker.  If we use the same pattern as 
> > X86, we can let GCC assign sections+attributes for us and only get one 
> > copy in the object file.
> > 
> > > Or is the linker not removing duplicates that some from the same source
> > > file?
> > 
> > In my testing, yes, the linker is merging them.
> 
> So, playing devil's advocate here, what is this fixing?

It's caused some confusion looking at object files, it's certainly not a 
critical bug!

> It doesn't really matter where in the toolchain the duplicates get
> removed, just that it happens somewhere...
> 
> It looks like ld -r doesn't do the deduplication, so there may be a fair
> amount of duplicates before the final linux of vmlinux -- which I guess
> could be an issue if BUG() is used in macros that get expanded all over
> the place.

Ah, interesting.  Testing with a loadable module, this means that there 
are duplicates in modules.  With mainline:

  strings ./fs/ext4/ext4.ko | grep -c "inode\.c"
  61

and with my patch:

  strings ./fs/ext4/ext4.ko | grep -c "inode\.c"
  4

Thanks,

Jamie