Re: [PATCH v2] RISC-V: Fix IFUNC resolver cannot access gp pointer

[Palmer Dabbelt <palmer@rivosinc.com>]

On Tue, 29 Oct 2024 19:58:14 PDT (-0700), cyy@cyyself.name wrote:
> Friendly ping.
>
> FYI, this bug can be reproduced through the steps outlined in the BZ #32269.
>
> Link: https://sourceware.org/bugzilla/show_bug.cgi?id=32269
>
> On 10/16/24 01:01, Yangyu Chen wrote:
>> In some cases, an IFUNC resolver may need to access the gp pointer to
>> resolve the function address. Such an object may have l_relocated == 0.
>> In this case, the GP register will not be set up. Thus, the IFUNC
>> resolver cannot access the gp pointer. This patch fixes this issue by
>> relaxing the check of l_relocated in elf_machine_runtime_setup.
>>
>> As for the original Bug 31317, since the static-linked executable has
>> already set up the gp pointer, we don't need to execute the code to set
>> up the gp pointer again. I have also reproduced and checked Bug 31317,
>> this patch can fix the issue.

We had a pretty similar issue with the hwprobe IFUNC stuff, and IIRC the 
result there was that it's just not safe to look up global symbols from 
an IFUNC.

>> Closes: BZ #32269
>> Fixes: 96d1b9ac23 ("RISC-V: Fix the static-PIE non-relocated object check")
>> Signed-off-by: Yangyu Chen <cyy@cyyself.name>
>> ---
>>   sysdeps/riscv/dl-machine.h | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/sysdeps/riscv/dl-machine.h b/sysdeps/riscv/dl-machine.h
>> index b2f28697f7..10a36d6701 100644
>> --- a/sysdeps/riscv/dl-machine.h
>> +++ b/sysdeps/riscv/dl-machine.h
>> @@ -348,7 +348,8 @@ elf_machine_runtime_setup (struct link_map *l, struct r_scope_elem *scope[],
>>         gotplt[1] = (ElfW(Addr)) l;
>>       }
>>
>> -  if (l->l_type == lt_executable && l->l_relocated)
>> +#ifdef SHARED
>> +  if (l->l_type == lt_executable)
>>       {
>>         /* The __global_pointer$ may not be defined by the linker if the
>>   	 $gp register does not be used to access the global variable
>> @@ -368,6 +369,7 @@ elf_machine_runtime_setup (struct link_map *l, struct r_scope_elem *scope[],
>>             : "r" (ref->st_value)
>>           );
>>       }
>> +#endif
>>   #endif
>>     return lazy;
>>   }