linux-kernel - Re: [PATCH] x86/kbuild: enable modversions for symbols exported from asm

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Date:   Wed, 14 Dec 2016 10:15:39 +0100
From:   Michal Marek <mmarek@...e.com>
To:     Dodji Seketeli <dodji@...eteli.org>
Cc:     Nicholas Piggin <npiggin@...il.com>,
        Ian Campbell <ijc@...lion.org.uk>,
        Ben Hutchings <ben@...adent.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Adam Borowski <kilobyte@...band.pl>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Debian kernel maintainers <debian-kernel@...ts.debian.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/kbuild: enable modversions for symbols exported from
 asm

On 2016-12-14 09:58, Dodji Seketeli wrote:
> Michal Marek <mmarek@...e.com> a écrit:
> 
> [...]
> 
>> Does the abidiff tool handle the case when an exported symbol is moved
>> between .c files? This is always a mess with genksyms, because the two
>> .c files have different includes and thus the type expansion stops at
>> different points. So typically the move needs to be reverted as a
>> workaround.
> 
> Let's consider the function:
> 
>   'void foo(struct S*);'
> 
> If two ELF binaries contain a definition of that function foo which ELF
> symbol is exported, if the type struct S hasn't changed, and if the only
> difference between the ELF binaries is that foo was defined in the
> translation unit a.c in the first binary and in b.c in the second
> binary, then the comparison engine of libabigail (which is the library
> that abidiff uses) will consider the declarations of the two foo
> functions as being equal -- no matter what include file comes before the
> definition point of foo in a.c and b.c.  If it does not, then it's a bug
> that ought to be fixed.
> 
> If you feel that I haven't understood your question, then I guess a
> minimal standalone example (in the form of C source code) that
> illustrates your use case could be helpful to me.

A minimal example would be

t1.c:
struct s1;
struct s2 {
	int i;
}
struct s3 {
	struct s1 *ptr1;
	struct s2 *ptr2;
}
void foo(struct s3*);
EXPORT_SYMBOL(foo);

t2.c:
struct s1 {
	int j;
}
struct s2;
struct s3 {
	struct s1 *ptr1;
	struct s2 *ptr2;
}
void foo(struct s3*);
EXPORT_SYMBOL(foo);

genksyms expands this to
void foo ( struct s3 { struct s1 { UNKNOWN } * ptr1 ; struct s2 { int i ; } * ptr2 ; } * )

or

void foo ( struct s3 { struct s1 { int j ; } * ptr1 ; struct s2 { UNKNOWN } * ptr2 ; } * )

respectively. The types are the same, but their visibility in the
different compilation units differs.

Michal