Re: [PATCH 1/5] vmlinux.lds.h: Include *(.text.*) in TEXT_TEXT

[James Bottomley]

On Mon, 2010-06-14 at 22:45 -0400, Tim Abbott wrote: 
> On Mon, 14 Jun 2010, James Bottomley wrote:
> 
> > > I believe that the pattern [A-Za-z$_] matches all valid characters to 
> > > start a function name (in particular, it includes "$").  If I'm missing 
> > > any valid characters for the start of a function name, please correct me.
> > 
> > Well, our linker seems to generate annoying symbols with carets in
> > them ...
> 
> The question here is: is there C code that when compiled with 
> -ffunction-sections will generate an ELF section with a name that starts 
> with ".text.^"?  For that to happen, you would need a function whose name 
> started with "^", which isn't valid C.

True ... but I didn't say that it was valid C.  I said the compiler is
spitting them out in the assembly ... I suspect it uses invalid C
function characters precisely to avoid clashes.

> The relevant namespace here is the names for ELF sections generated by 
> -ffunction-sections.  These are in turn computed by the compiler from 
> function names -- there's no potential conflict created by 
> linker-generated symbols whose names start with a caret.  Similarly, for 
> -fdata-sections, we only care about the names of C data objects, which 
> also can't start with a caret.
> 
> > > While one could in principle try to handle things by not renaming the 
> > > .text.foo sections and instead just placing the linker script code for 
> > > them all before a .text.* item in the linker script, that approach is 
> > > really fragile.  I think the "text..foo" approach is a good design and I 
> > > am not aware of any problems with it.
> > 
> > OK, but how about some actual explanation?  You've just characterised
> > the current -ffunction-sections scheme that parisc has used for decades
> > as "fragile"
> 
> The current parisc situation is fine.
> 
> What I was trying to draw a contrast with is supporting -fdata-sections by 
> adding ".data.*" to DATA_DATA, and then trying to make sure that all the 
> architecture linker scripts handle all the kernel's special data sections 
> with names like ".data.foo" before the place where DATA_DATA appears in 
> their linker scripts.  Most of the architecture linker scripts mention 
> more than a half dozen special kernel sections with names of the form 
> ".data.foo", often in fairly random orders, and so it would be really 
> fragile to add the constraint that these sections need to all appear above 
> DATA_DATA.
> 
> Adding ".data.[A-Za-z$_]" to DATA_DATA doesn't have this problem.
> 
> If we similarly added ".text.[A-Za-z$_]" to TEXT_TEXT, we'd presumably 
> move the 4 named .text.foo sections before TEXT_TEXT; I don't think any 
> other architectures would require any work.
> 
> > > Some more detailed explanation is available here:
> > > <http://lkml.org/lkml/2010/2/19/365>
> > 
> > That's still a bit short on explanations.
> >
> > But if I infer from the rest, someone, somewhere broke the convention
> > that all our special linux sections be called .XX.data and .XX.text to
> > distinguish them from the .text.FF and .data.YY the compiler will
> > generate with the relevant sectional directives? because it's been
> > working OK for us for a while.
> 
> I don't know the full history here.  But prior to the ".data.foo" -> 
> ".data..foo" patches that were merged recently, there were a bunch of 
> cross-architecture sections with these sorts of names, e.g.:
> 
> .data.page_aligned
> .data.nosave
> .data.read_mostly
> .data.cacheline_aligned
> .data.lock_aligned
> .data.percpu*
> .data.init_task
> etc.
> 
> There were also a bunch of ".text.foo" sections on individual 
> architectures, many of which currently don't support -ffunction-sections 
> (sh, ia64, x86, mips, etc.).
> 
> However, there weren't any .text.foo sections that are cross-architecture.  
> Since parisc only uses -ffunction-sections, and not -fdata-sections, the 
> popular .data.foo naming scheme doesn't cause any breakage on parisc.
> 
> The only architecture that does use -fdata-sections is frv, and there 
> could theoretically have been breakage there, but in practice it's likely 
> nobody has written kernel code that would actually conflict, e.g. "static 
> int percpu = 3;", yet.

Right, but what I was curious about, since, we already sorted all the
problems with text sections out on parisc by putting the .text as a
suffix not a prefix, and you have to change all the data sections
anyway, why not just follow established practise?

> > To fix the breakage, the proposal now is to name all linux special
> > sections as .text..XX and .data..XX?  I can see that's more standard
> > looking that XX.text and XX.data, but not necessarily that it's better.
> 
> Yes, that's the proposal.
> 
> > This then introduces a problem of matching because .text.X and .text..X
> > are hard to distinguish using the linker matching scripts.
> 
> Right.  I believe that this is totally solvable with a simple linker 
> script pattern, since the space of valid names for functions and data 
> objects in C code is quite restricted (and that the implementation of 
> using e.g. ".data.[A-Za-z$_]*" solves this problem).
> 
> > So even if I buy the rename of the linux symbols, what about using a 
> > linker defined symbol that's illegal as a function as the initial 
> > separator instead of .?  So hyphen looks the obvious one ... you can 
> > have all the linux special sections being .text- and .data- then we can 
> > easily distinguish.
> 
> Is "." a valid first character for a function name?  I don't see the 
> problem with using "." here.

The problem is that it's hard to get the linker to distinguish
between .text.. and .text. without funny regexp contortions.  However,
if the two sections began .text- and .text. they are easy to distinguish
because one prefix isn't a subset of the other.

> Both .page_aligned.data and .data-page_aligned are valid choices (and in 
> fact, the first patch series I sent on this topic about 18 months ago did 
> .page_aligned.data, I think).
> 
> The main technical difference between ".data..page_aligned" and 
> ".page_aligned.data" in my view is that you need to be more careful when 
> writing assembly files with ".page_aligned.data".
> 
> To give an example, if I run the following:
> 
> $ cat > foo.s 
> .section .data-page-aligned
> 	.long 0
> .section .data.page_aligned
> 	.long 1
> $ gcc -c foo.s -o foo.o
> $ objdump -h foo.o 
> foo.o:     file format elf32-i386
> 
> Sections:
> Idx Name          Size      VMA       LMA       File off  Algn
>   0 .text         00000000  00000000  00000000  00000034  2**2
>                   CONTENTS, ALLOC, LOAD, READONLY, CODE
>   1 .data         00000000  00000000  00000000  00000034  2**2
>                   CONTENTS, ALLOC, LOAD, DATA
>   2 .bss          00000000  00000000  00000000  00000034  2**2
>                   ALLOC
>   3 .data-page-aligned 00000004  00000000  00000000  00000034  2**0
>                   CONTENTS, READONLY
>   4 .data.page_aligned 00000004  00000000  00000000  00000038  2**0
>                   CONTENTS, ALLOC, LOAD, DATA
> 
> one can see that the .data-page-aligned section doesn't have the right 
> section flags.  So I'm pretty sure the relevant assembler heuristic is 
> looking for things starting with ".data.", not just ".data".
> 
> The kernel has a lot of code in assembly files that just does:
> 
> .section ".data"
> 
> and so there's a very real risk that folks who are doing pattern-matching 
> development on assembly files will end up creating non-allocated sections 
> by accident (I've certainly made this mistake myself, and there's a bug of 
> this form in arch/x86/lib/thunk.S until commit 
> c6c2d7a084d14a8a701be84872aa1b77d2945f46, so I don't think I'm alone)

But that commit isn't anything to do with the section not being
allocated.  That's actually irrelevant to us, since we sort out the
sectional allocations out at link time with the linker script (which
ends up completely ignoring the original file flags).  The problem,
specifically, is that the linker does a rename of two identically named
sections with different flags ... we would also get the same behaviour
if the linker thought two sections weren't mergeable even if they did
have identical flags.

In general, I think it's good practise for all use of sections to
specify the elf flags.

> I also think that ".data..page_aligned" is more readable as a new name for 
> the former ".data.page_aligned" than ".page_aligned.data" is, but I think 
> that's a secondary consideration.  ".data.-page_aligned" would be 
> technically equivalent to ".data..page_aligned", but I think it is uglier.

Actually, as I said, that would be .data- 

But, to be honest, I don't care that much about the data names because
we don't use -fdata-sections, the only objection is that having two
separate conventions for text and data is adding needless complexity ...
I do care about the text names, and I'd rather we stuck to the existing
convention there, or provided a good reason for the change ... and
wanting .text. as a prefix for everything is weak at best.

The specific objection I have to converting everything to the .text..*
prefix for linux sections is that the gather all function sections can
no longer be *(.text.*), but has to become a regular expression, which
is adding fragility.

James


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