[AArch64] Fix pointer inequality in pointers to the same ifunc symbol

[parth-07]

This commit fixes pointer inequality in pointers to the same ifunc symbol. Let's understand the issue in more detail using the below example:

char *foo_impl() { return "foo"; }

static char *(*foo_resolver())(void) { return foo_impl; }

char *foo() __attribute__((ifunc("foo_resolver")));

char *(*foogp)() = foo;

int main() {
  char *(*foop)() = foo;
}

foogp and foop pointers both point to the same function foo. However, their values were different. The root cause was improper handling of the underlying relocations when the symbol is of type ifunc.

char *(*foogp)() = foo; // R_AARCH64_ABS64
// ...
char *(*foop)() = foo; // R_AARCH64_ADR_GOT_PAGE + R_AARCH64_LD64_GOT_LO12_NC

Previously, the linker resolved the R_AARCH64_ABS64 relocation here to the plt slot of foo, let's refer to it as plt[foo], and the GOT-relocations were resolved to the .got.plt slot of foo, let's refer to it as .got.plt[foo]. As a result, foogp stores the address of plt[foo] and foop stores the contents of .got.plt[foo], that is, the address of the resolved function foo. Clearly, the two values are different.

We resolve this issue by creating a got slot for foo when the foo has both an absolute reference and a GOT-reference. The got slot of foo is filled by the linker and stores the address of plt[foo]. With this, both the absolute-reference and got-reference to an ifunc symbol results in the same address. Please note that:

• we do not create .got slot of foo when we only have an absolute reference to foo because .got slot is unnecessary in this case, and
• we do not create .got slot of foo when we only have a GOT-reference because in this case we can directly use .got.plt slot and access the function directly without any indirection penalty or the pointer-inequality bug.

Resolves #913

[quic-seaswara]

The GNU linker creates a igot and igot.plt probably it would be useful to follow the pattern for IFUNC symbols.

[quic-seaswara]

This can simplify the ifunc implementation too, probably we can reserve the GOT and PLT slots and later remove the GOT slots not needed.