Re: [PATCH] x86: clean up vdso-layout.lds.S

[Sam Ravnborg <sam@ravnborg.org>]

On Tue, Jun 09, 2009 at 09:26:58AM +0200, Petr Tesarik wrote:
> Sam Ravnborg píše v Pá 05. 06. 2009 v 22:07 +0200:
> > > > diff --git a/arch/x86/vdso/Makefile b/arch/x86/vdso/Makefile
> > > > index 16a9020..8c7f06a 100644
> > > > --- a/arch/x86/vdso/Makefile
> > > > +++ b/arch/x86/vdso/Makefile
> > > > @@ -23,7 +23,8 @@ $(obj)/vdso.o: $(obj)/vdso.so
> > > >  
> > > >  targets += vdso.so vdso.so.dbg vdso.lds $(vobjs-y)
> > > >  
> > > > -export CPPFLAGS_vdso.lds += -P -C
> > > > +vdso-cppflags = -P -C
> > > > +export CPPFLAGS_vdso.lds += -m64 $(vdso-cppflags)
> > 
> > I am wondering why we need -P -C here - but we do not need it for lds.S files?
> > Seems like something we could let go.
> 
> Frankly, I don't know, but it's been there for ages, and I don't see
> what you mean, anyway. Doesn't the top-level Makefile contain this line,
> for example:
> 
> export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH)

I had forgotten about the top-level Makefile setting these.
They should have moved to Makefile.build long time ago :-(

Keep them here.

> 
> > > >  VDSO_LDFLAGS_vdso.lds = -m elf_x86_64 -Wl,-soname=linux-vdso.so.1 \
> > > >  		      	-Wl,-z,max-page-size=4096 -Wl,-z,common-page-size=4096
> > > > @@ -68,7 +69,7 @@ vdso32.so-$(VDSO32-y)		+= sysenter
> > > >  
> > > >  vdso32-images			= $(vdso32.so-y:%=vdso32-%.so)
> > > >  
> > > > -CPPFLAGS_vdso32.lds = $(CPPFLAGS_vdso.lds)
> > > > +CPPFLAGS_vdso32.lds = -m32 $(vdso-cppflags)
> > > >  VDSO_LDFLAGS_vdso32.lds = -m elf_i386 -Wl,-soname=linux-gate.so.1
> > > >  
> > > >  # This makes sure the $(obj) subdirectory exists even though vdso32/
> > > > diff --git a/arch/x86/vdso/vdso-layout.lds.S b/arch/x86/vdso/vdso-layout.lds.S
> > > > index 634a2cf..1f4b215 100644
> > > > --- a/arch/x86/vdso/vdso-layout.lds.S
> > > > +++ b/arch/x86/vdso/vdso-layout.lds.S
> > > > @@ -22,16 +22,15 @@ SECTIONS
> > > >  	.eh_frame	: { KEEP (*(.eh_frame)) }	:text
> > > >  
> > > >  	.dynamic	: { *(.dynamic) }		:text	:dynamic
> > > > +	.got		: { *(.got.plt) *(.got) }	:text
> > 
> > The style we try to introduce for .lds files in
> > arch/$ARCH/kernel/vmlinux.lds.S is much more C-like.
> > The above would have been:
> > 	.got : {
> > 		*(.got.plt)
> > 		*(.got)
> > 	} :text
> > 
> > Please use this all over so we have a consistent style in linker scripts.
> 
> OK, so should I first post a patch which doesn't change anything but
> adjusts the coding style of vdso-layout.lds.S?

That would be great.
Then your following changes would be easier to read/review.

> 
> 
> > > >  	.data		: {
> > > > -	      *(.data*)
> > > > -	      *(.sdata*)
> > > > -	      *(.got.plt) *(.got)
> > > > -	      *(.gnu.linkonce.d.*)
> > > > -	      *(.bss*)
> > > > -	      *(.dynbss*)
> > > > -	      *(.gnu.linkonce.b.*)
> > > > +		*(.data .data.* .gnu.linkonce.d.*)
> > > > +		*(.bss .bss.* .gnu.linkonce.b.*)
> > > > +		*(COMMON)
> > > >  	}
> > Where did *(.sdata*) go?
> 
> Nothing changed, really. This section is never produced for i386 or
> x86_64. The line might have been a remnant of the (long abandoned) idea
> that x86_64 would have .sdata and .data, but it then turned to be
> rather .data and .ldata.
> 
> > Why do we need *(.data .data.*) rather than *(.data*)?
> > *(.dynbss*)?
> 
> I don't have any strong opinion here, but the former is exactly what the
> default linker script has.

My general take on this is that we should know and deal with what the linker produces.
So if we only expect .data then .data.* could go into .broken so we catch it. 

> 
> > In your changelog you say:
> > "discard sections which are not useful to user-space" - but as you do not
> > list which one it is hard to tell what you removed on purpose
> > and what you removed by accident.
> 
> All those which end up in the section ".broken". I'll make a better
> wording next time.
> 
> > > >  }
> > > >  
> > > > +ASSERT(!SIZEOF(.broken), "VDSO contains sections that don't work properly");
> > Can you give any better hints where to look and what to look for?
> 
> What would you expect? The linker script language is quite limited in
> its capabilities... Best I could do is split the ".broken" section into
> several sections and move the descriptions from the individual comments
> above here. If this muckle of empty ".broken.*" sections gets correctly
> discarded and triggers no bug in binutils, I can probably do it.

I was more think of something like this:

+/*
+ * This assert is triggered if the linker produces a non-empty section
+ * that is listed in the .broken section.
+ * Use objdump -h to see which is the offending section
+ */
+ASSERT(!SIZEOF(.broken), "The vdso linker script found a section that is bad. See xx.lds for details");


> > > > +/* Check that GOT has only the three entries defined by the ABI */
> > > > +ASSERT(SIZEOF(.got) == 3*__SIZEOF_POINTER__,
> > > > +	"Found extra GOT entries. Check your use of external vars.");
> > Can you give any better hints where to look and what to look for?
> 
> What would you consider a better hint? If there are any entries in the
> GOT, this means that the vDSO tries to access an external function or
> variable. But since we don't have a real in-kernel linker for the vDSO,
> these references will remain unresolved (and most likely cause a
> segmentation fault at run-time).
And here reword the above explanation a bit and give a hint
how to see what is there that is unexpected.
And stuff it in a comment.

	Sam