Re: [llvm-dev] RFC: Add GNU_PROPERTY_1_NEEDED

["H.J. Lu via Libc-alpha" <libc-alpha@sourceware.org>]

On Fri, Jun 18, 2021 at 2:34 PM Fangrui Song <maskray@google.com> wrote:
>
> On 2021-06-18, H.J. Lu via llvm-dev wrote:
> >Add GNU_PROPERTY_1_NEEDED:
> >
> > #define GNU_PROPERTY_1_NEEDED      GNU_PROPERTY_UINT32_OR_LO
> >
> >to indicate the needed properties by the object file.
> >
>
> I am fine with this logical OR style usage. But see below, do we need it
> for ld.so runtime check?

I implemented run-time check on users/hjl/single-global/master branch:

https://gitlab.com/x86-glibc/glibc/-/commits/users/hjl/single-global/master

with tests:

[hjl@gnu-cfl-2 build-x86_64-linux]$ elf/tst-protected1a
copy relocation against non-copyable protected symbol=protected1 in
file=/export/build/gnu/tools-build/glibc-cet-gitlab/build-x86_64-linux/elf/tst-protected1moda.so
[hjl@gnu-cfl-2 build-x86_64-linux]$ elf/tst-protected2a
`protected1' in main and moda doesn't have the same address
non-canonical reference to canonical protected function
symbol=protected1 in
file=/export/build/gnu/tools-build/glibc-cet-gitlab/build-x86_64-linux/elf/tst-protected2moda.so
[hjl@gnu-cfl-2 build-x86_64-linux]$

I prefer these over random run-time failures.

> (As I mentioned previously, I do not know how an AND-style property can
> be used/deployed if old object files without the .note.gnu.property is
> considered to have a value of 0.)
>
> >Add GNU_PROPERTY_1_NEEDED_SINGLE_GLOBAL_DEFINITION:
> >
> > #define GNU_PROPERTY_1_NEEDED_SINGLE_GLOBAL_DEFINITION  (1U << 0)
> >
> >to indicate that the object file requires canonical function pointers and
> >cannot be used with copy relocation.
>
> In https://sourceware.org/pipermail/gnu-gabi/2021q2/000481.html you gave
> a rationale
>
> "The issue is that libfoo.so used in link-time can be different from
>   libfoo.so at run-time.   The symbol, foobar, in libfoo.so at link-time
>   has the default visibility.  But foobar in libfoo.so at run-time can be
>   protected.  ld.so should detect such cases which can lead to run-time
>   failures."
>
> First, I think such dynamic symbol visibility change is uncommon.

I can imagine that some libraries want to switch to protected symbols.

> Second, if ld.so finds that a symbol lookup for (st_value==0
> st_shndx==SHN_UNDEF) will bind to a STV_PROTECTED definition in a shared
> object, can the diagnostic be moved there?
> The compatibility property is per-symbol and the symbol lookup is a
> perfect place for a diagnostic, like a symbol versioning error.
>
>
> I guess GCC folks may get noticed if you start a thread adding
> -fsingle-global-definition, otherwise many people who have opinions may
> just ignore threads about GNU PROPERTY addition.

Binutils changes are at

https://gitlab.com/x86-binutils/binutils-gdb/-/tree/users/hjl/property/master

GCC changes are next.

> >
> >The PDF file is at
> >
> >https://gitlab.com/x86-psABIs/Linux-ABI/-/wikis/uploads/9eca2f2defe62b0c5015bf2e3e8a9f05/Linux-gABI-1_needed-2021-06-18.pdf



-- 
H.J.