https://sourceware.org/bugzilla/show_bug.cgi?id=32443
Bug ID: 32443
Summary: ABS symbol value depends on its visibility
Product: binutils
Version: 2.40
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: ld
Assignee: unassigned at sourceware dot org
Reporter: srk31 at srcf dot ucam.org
Target Milestone: ---
Created attachment 15835
--> https://sourceware.org/bugzilla/attachment.cgi?id=15835&action=edit
reproducer
If I define an ABS symbol preemptibly in a shared object, such that its GOT
entry is relocated by the ld.so and a `R_*_GLOB_DAT` relocation, the ABS symbol
takes the value I expect.
```
$ grep . exe.c lib.c lib.lds
exe.c:extern void lib_func(void);
exe.c:int main(void)
exe.c:{
exe.c: lib_func();
exe.c:}
lib.c:#include <stdio.h>
lib.c:extern void value;
lib.c:void lib_func(void)
lib.c:{
lib.c: printf("In lib, value is %lx\n", (long) &value);
lib.c:}
lib.lds:PROVIDE (value = 42);
$ cc -fPIC -shared -o lib.so lib.c lib.lds -Wl,-rpath,`pwd`
$ cc -fPIC -o exe exe.c lib.so -Wl,-rpath,`pwd`
$ ./exe
exe: In lib, value is 2a
```
But if I make the same symbol non-preemptible, without changing its value (and
noting that it is *not* preempted in this scenario), its value gets the
library's load address added to it.
```
$ grep . lib-hidden.lds
PROVIDE_HIDDEN (value = 42);
$ cc -fPIC -shared -o lib-hidden.so lib.c lib-hidden.lds -Wl,-rpath,`pwd`
$ cc -fPIC -o exe-hidden exe.c lib-hidden.so -Wl,-rpath,`pwd`
$ ./exe-hidden
exe-hidden: In lib, value is 7fe650c5c02a
```
The problem is that the GOT entry is now relocated differently: by
`R_*_RELATIVE`, not `R_*_GLOB_DAT`. Once `R_*_RELATIVE` is applied, of course
the load address will inevitably be added.
I suspect that for ABS symbols, these `R_*_RELATIVE` relocations on the GOT
entries should not be generated. This would generate behaviour consistent with
the `R_*_GLOB_DAT` case, giving the ABS section's address as (always) "zero in
the global address space", not sometimes-yes-sometimes-not "zero in the shared
object's address space".
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