Implement arm64 TLSDESC

Each TLSDESC relocation relocates a 2-word descriptor in the GOT that
contains:
 - the address of a TLS resolver function
 - an argument to pass (indirectly) to the resolver function

(Specifically, the address of the 2-word descriptor is passed to the
resolver.)

The loader resolves R_GENERIC_TLSDESC relocations using one of three
resolver functions that it defines:
 - tlsdesc_resolver_static
 - tlsdesc_resolver_dynamic
 - tlsdesc_resolver_unresolved_weak

The resolver functions are written in assembly because they have a
restrictive calling convention. They're only allowed to modify x0 and
(apparently) the condition codes.

For a relocation to memory in static TLS (i.e. the executable or an solib
loaded initially), the loader uses a simple resolver function,
tlsdesc_resolver_static, that returns the static offset it receives from
the loader.

For relocations to dynamic TLS memory (i.e. memory in a dlopen'ed solib),
the loader uses tlsdesc_resolver_dynamic, which allocates TLS memory on
demand. It inlines the fast path of __tls_get_addr, then falls back to
__tls_get_addr when it needs to allocate memory. The loader handles these
dynamic TLS relocations in two passes:
 - In the first pass, it allocates a table of TlsDynamicResolverArg
   objects, one per dynamic TLSDESC relocation.
 - In the second pass, once the table is finalized, it writes the
   addresses of the TlsDynamicResolverArg objects into the TLSDESC
   relocations.

tlsdesc_resolver_unresolved_weak returns a negated thread pointer so that
taking the address of an unresolved weak TLS symbols produces NULL.

The loader handles R_GENERIC_TLSDESC in a target-independent way, but
only for arm64, because Bionic has only implemented the resolver functions
for arm64.

Bug: http://b/78026329
Test: bionic unit tests
Test: check that backtrace works inside a resolver function and inside
  __tls_get_addr called from a resolver
  (gdbclient.py, b __tls_get_addr, bt)
Merged-In: I752e59ff986292449892c449dad2546e6f0ff7b6
Change-Id: I752e59ff986292449892c449dad2546e6f0ff7b6

0 files changed, 0 insertions, 0 deletions