Re: swsusp.S: meaningfull assembly labels

["Richard B. Johnson" <root@chaos.analogic.com>]

On Fri, 25 Jun 2004, Daniel Jacobowitz wrote:

> On Fri, Jun 25, 2004 at 12:02:47PM -0400, Richard B. Johnson wrote:
> > On Fri, 25 Jun 2004, Daniel Jacobowitz wrote:
> >
> > > On Fri, Jun 25, 2004 at 08:37:45AM -0400, Richard B. Johnson wrote:
> > > > NO! You just made those labels public! The LOCAL symbols need to
> > > > begin with ".L".  Now, if you have a 'copy_loop' in another module,
> > > > linked with this, anywhere in the kernel, they will share the
> > > > same address -- not what you expected, I'm sure! The assembler
> > > > has some strange rules you need to understand. Use `nm` and you
> > > > will find that your new labels are in the object file!
> > >
> > > Er, no.  They'll show up in the object file.  That doesn't mean they're
> > > global; static symbols also show up in the object file.
> > >
> > > --
> > > Daniel Jacobowitz
> >
> > I got caught on these, thinking that they weren't globals when
> > I made a assembly files that used, not only named labels but
> > also definitions like:
> >
> > BUF = 0x08
> > LEN = 0x0c
> >
> > code:	movl	BUF(%esp), %ebx
> > 	movl	LEN(%esp), %ecx
> >
> >
> > This was done in several files. When linked, I got 'duplicate synbol'
> > errors. These symbols, while not 'globals' in the sense that 'C'
> > code can link with them are global definitions that are seen by
> > the linker and cause duplicate symbol errors. I went through
>
> Those are what gas calls absolute symbols.
>
> > the gas documentation, trying to find how to prevent these
> > private symbols from being written to the object file and there
> > is no command-line mechanism. You just have to start every name
> > with .L to keep them local.
> >
> > When I used labels like 'code' above, even though not declared
> > '.global',  the labels also showed up as duplicate symbols when
> > linking the resulting object files.
>
> Does this only happen in your world?  Or were you using a broken
> non-GNU linker?
>

It is the GNU linker

> drow@nevyn:~% cat a.s
> BUF = 0x1
> code:
>         mov %eax, 0
> drow@nevyn:~% as -o a.o a.s
> drow@nevyn:~% as -o b.o a.s
> drow@nevyn:~% ld -o a a.o b.o
> ld: warning: cannot find entry symbol _start; defaulting to 0000000008048074
> drow@nevyn:~% nm a.o
> 00000001 a BUF
> 00000000 t code
> drow@nevyn:~% nm a
> 00000001 a BUF
> 00000001 a BUF
> 08049084 A __bss_start
> 08049084 A _edata
> 08049084 A _end
> 08048074 t code
> 0804807c t code
>
> LD won't even warn about duplicate absolute symbols unless they have
> different values.
>

But they ARE different values. They will probably always be different
values, and they end up with the same name, that `ld` sees and complains.


> --
> Daniel Jacobowitz
>

Cheers,
Dick Johnson
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