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Re: rtld: resolve incoming STT_GNU_IFUNC only after DT_INIT
On 11/01/2017 1724Z, Florian Weimer wrote:
* John Reiser:
However, a relocation from some other module to a STT_GNU_IFUNC symbol
in the current shared library cannot be intercepted like that.
That's why DT_INIT should be called before such a relocation is resolved,
so that the IFUNC code can be de-compressed into the right place
before it gets called.
I don't think it's safe at all to call DT_INIT during relocation
processing. You need to explain why you think it is.
If all processing is done in dependency order (thus "B depends on A"
requires that all processing of A occurs before any processing of B;
this includes "B has a relocation that designates a symbol that is
resolved by A with type STT_GNU_IFUNC")
*and* if internal STT_GNU_IFUNC resolution within the same module
is hidden from rtld like I have suggested, then calling the DT_INIT
will be safe. All the STT_GNU_IFUNC symbols in predecessor modules
in the dependency order will be available, and all the STT_GNU_IFUNC
symbols in the same current module will be specially processed (transformed
into R_*_IRELATIVE by /bin/ld, hidden from rtld by the compressor, and
resolved by DT_INIT as soon as their instructions have been de-compressed
and before any "normal" execution begins.)
I really don't see how we can make this happen. As I said, DT_INIT
may need the relocations which result from IFUNC processing.
If those IFUNC-backed relocations are in predecessor modules (topological
sort of dependencies) then they will have been taken care of already
because *all* processing of predecessors occurs first:
Process the relocations of module A, call A.DT_INIT.
Process the relocations of module B, call B.DT_INIT.
If those IFUNC-backed relocations are in the current module
then they will be handled by STT_GNU_IFUNC ==> R_*_IRELATIVE
==> compressor hides from rtld ==> DT_INIT calls IFUNC immediately
after de-compression.
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