Re: [PATCH] arm64/vdso: Remove unused makefile variable

[Will Deacon <will@kernel.org>]

On Tue, Apr 28, 2020 at 01:52:55PM +0200, Ard Biesheuvel wrote:
> On Tue, 28 Apr 2020 at 12:45, Vincenzo Frascino
> <vincenzo.frascino@arm.com> wrote:
> > On 4/27/20 9:35 PM, Will Deacon wrote:
> > > [+Vincenzo]
> > >
> > > On Fri, Apr 24, 2020 at 08:58:49AM -0700, Geoff Levand wrote:
> > >> The vdso makefile variable VDSO_LDFLAGS is defined, but never used,
> > >> so remove it.
> > >>
> > >> Signed-off-by: Geoff Levand <geoff@infradead.org>
> > >> ---
> > >>
> > >> Hi,
> > >>
> > >> This seems to be left over from a code cleanup that missed it.
> > >
> > > While I agree that this isn't used, I'm wondering why '-Bsymbolic' is used
> > > to link the compat vDSO but not the native one. It seems weird to differ
> > > in this regard.
> > >
> > > Vincenzo? Looks like you added this unused variable in 28b1a824a4f44
> > > ("arm64: vdso: Substitute gettimeofday() with C implementation").
> > >
> >
> > My understanding is that "-Bsymbolic" is required by both compat and normal vdso
> > because when the shared library is built it adds a flag in the dynamic section
> > of the binary called DT_SYMBOLIC which alters the dynamic linker's symbol
> > resolution algorithm to search for references for a symbol inside the library
> > first and then into the executable.
> >
> 
> DT_SYMBOLIC doesn't (or shouldn't) change the dynamic linking
> behavior. It informs the linker that ELF symbol preemption may not
> work, since the .so has bound internal references to its exported
> symbols to the internal versions directly, rather than allowing the
> application to supersede (i.e., 'preempt') them. This is an obscure
> feature that isn't really relevant for the VDSO, since we carefully
> control what we export from the .so anyway (via the linker script's
> VERSIONS section)
> 
> > This becomes useful for example when an executable built with -fPIC is trying to
> > call a public vDSO function from assembly (bl symbol).
> >
> > The issue here seems to be that I used VDSO_LDFLAGS instead of ldflags-y. I can
> > post a patch and Cc stable. Adding Geoff as Reported-by.
> >
> 
> I think it can be removed.

Hmm, so I did a little bit more digging because -Bsymbolic is used to link
the vDSO on arm, mips, sparc and x86. Commit 6f121e548f83 ("x86, vdso:
Reimplement vdso.so preparation in build-time C") suggests that it's a good
idea to prevent any unexpected dynamic relocations appearing in the vDSO
object.

Will

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