https://github.com/smithp35 commented:
How does this work in the non-PIE (PDE) case when we take the address of an
ifunc and pass it to a function in a shared library, which then compares the
argument with its own global address take of the ifunc?
For example:
shared lib
```
typedef void Fptr(void);
extern void ifn(void);
// take address of ifunc ifn defined in application
Fptr* ifp = &ifn;
// compare address of ifn we have calculated in ifp vs
// address calculated by application, passed in fp1.
int compare(Fptr* fp1) {
return fp1 == ifp;
}
```
App
```
typedef void Fptr(void);
extern int compare(Fptr* fp1);
int val = 0;
static void impl(void) { val = 42; }
static void *resolver(void) { return impl; }
__attribute__((ifunc("resolver"))) void *ifn();
extern Fptr* fp;
int main(void) {
return compare(fp);
}
// separate file so compiler is unaware ifn is an ifunc.
typedef void Fptr(void);
extern void ifn(void);
Fptr* fp = &ifn;
```
Right now in the application lld produces an iPLT entry for `ifn`, with `fp`
pointing to the iPLT entry. The dynamic symbol table contains the address of
the iPLT entry with type STT_FUNC . The shared library and the argument compare
equal.
As I understand it, this patch will change `fp` to point directly to the result
of the ifunc resolver. So unless we also change the value put into the dynamic
symbol table we'll stop comparing equal.
I don't think there's a STT_FUNC symbol we can put in the dynamic symbol table
that holds the result of the ifunc resolver. GNU ld, puts the address of the
resolver function with a STT_GNU_IFUNC symbol type in the dynamic symbol table.
If that causes the dynamic loader to call the resolver and replace the value
with the result then that would work. I haven't had time to check what glibc
does though.
I'll put some more general comments below. Didn't want to make this one too
long.
https://github.com/llvm/llvm-project/pull/133531
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