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Re: [PATCH] PR ld/19636: pie changes program behavior and generate unnecessary dynamic symbols
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Alan Modra <amodra at gmail dot com>
- Cc: Binutils <binutils at sourceware dot org>
- Date: Sun, 21 Feb 2016 19:05:48 -0800
- Subject: Re: [PATCH] PR ld/19636: pie changes program behavior and generate unnecessary dynamic symbols
- Authentication-results: sourceware.org; auth=none
- References: <20160215150032 dot GA26273 at gmail dot com> <20160217052322 dot GA14941 at gmail dot com> <CAMe9rOoOKPsrxf51pzBzouREZYP0TUyFNq2XTZGY61p3kzrEgA at mail dot gmail dot com> <20160218004245 dot GG31757 at bubble dot grove dot modra dot org> <CAMe9rOqhYY6R7gUgaw7L5ab-Rs=3VDCVdBEsGcGCWde4aA6a8Q at mail dot gmail dot com> <20160218063124 dot GA10657 at bubble dot grove dot modra dot org> <CAMe9rOoKfOTQ5jT10oS1HBXMSBfhB2h9Me91_briyzD2GWScHw at mail dot gmail dot com> <20160222025308 dot GD10657 at bubble dot grove dot modra dot org>
On Sun, Feb 21, 2016 at 6:53 PM, Alan Modra <email@example.com> wrote:
> On Sat, Feb 20, 2016 at 09:59:24PM -0800, H.J. Lu wrote:
>> On Wed, Feb 17, 2016 at 10:31 PM, Alan Modra <firstname.lastname@example.org> wrote:
>> > On Wed, Feb 17, 2016 at 08:24:06PM -0800, H.J. Lu wrote:
>> >> On Wed, Feb 17, 2016 at 4:42 PM, Alan Modra <email@example.com> wrote:
>> >> > This change is incorrect. Some targets (even x86_64 with -fPIC)
>> >> > support
>> >> > if (fun)
>> >> > fun ();
>> >> > for an undefined weak fun at link time, making fun dynamic and
>> >> > emitting dynamic relocs against fun. So when the executable is linked
>> >> > against a new shared library that provides fun, fun will be called.
>> >> Yes, it should be moved before
>> > No, that change should not be added at all!
>> I moved it to x86 backend via elf_backend_fixup_symbol.
> Where it is just as wrong as anywhere else. You are changing the
> behaviour of the following program.
> cat > undefweak.c <<EOF
> extern void __attribute__ ((weak)) fun (void);
> int main (void)
> if (&fun != 0)
> fun ();
> return 0;
> gcc -c -fPIC -O2 undefweak.c
> gcc -o undefweak undefweak.o
> If some later version of libc.so defines "fun", then with your change,
> "fun" won't be called. I can't cite real-world examples of this
> behaviour off the top of my head, where "fun" is likely to be some
> function multiple levels deep in a tree of user shared libraries, but
> I'm almost certain you will break some people's expectations.
I am very surprised to see PIC in relocatable input make a difference
in output. I will ask on gABI list.