[llvm-dev] RFC: Add GNU_PROPERTY_1_NEEDED
H.J. Lu via llvm-dev
llvm-dev at lists.llvm.org
Fri Jun 18 18:09:42 PDT 2021
On Fri, Jun 18, 2021 at 2:34 PM Fangrui Song <maskray at 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:
[hjl at gnu-cfl-2 build-x86_64-linux]$ elf/tst-protected1a
copy relocation against non-copyable protected symbol=protected1 in
[hjl at 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
[hjl at 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
> 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
GCC changes are next.
> >The PDF file is at
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