> On Jul 24, 2024, at 06:50, Konstantin Belousov <k...@freebsd.org> wrote:
>
> On Wed, Jul 24, 2024 at 12:34:57PM +0200, m...@freebsd.org wrote:
>>
>>
>> On 24.07.2024 12:24, Konstantin Belousov wrote:
>>> On Tue, Jul 23, 2024 at 08:11:13PM +0000, John F Carr wrote:
>>>> On Jul 23, 2024, at 13:46, Michal Meloun <meloun.mic...@gmail.com> wrote:
>>>>>
>>>>> On 23.07.2024 11:36, Konstantin Belousov wrote:
>>>>>> On Tue, Jul 23, 2024 at 09:53:41AM +0200, Michal Meloun wrote:
>>>>>>> The good news is that I'm finally able to generate a working/locking
>>>>>>> test case. The culprit (at least for me) is if "-mcpu" is used when
>>>>>>> compiling libthr (e.g. indirectly injected via CPUTYPE in
>>>>>>> /etc/make.conf).
>>>>>>> If it is not used, libthr is broken (regardless of -O level or
>>>>>>> debug/normal
>>>>>>> build), but -mcpu=cortex-a15 will always produce a working libthr.
>>>>>> I think this is very significant progress.
>>>>>> Do you plan to drill down more to see what is going on?
>>>>>
>>>>> So the problem is now clear, and I fear it may apply to other
>>>>> architectures as well.
>>>>> dlopen_object() (from rtld_elf),
>>>>> https://cgit.freebsd.org/src/tree/libexec/rtld-elf/rtld.c#n3766,
>>>>> holds the rtld_bind_lock write lock for almost the entire time a new
>>>>> library is loaded.
>>>>> If the code uses a yet unresolved symbol to load the library, the
>>>>> rtl_bind() function attempts to get read lock of rtld_bind_lock and a
>>>>> deadlock occurs.
>>>>>
>>>>> In this case, it round_up() in _thr_stack_fix_protection,
>>>>> https://cgit.freebsd.org/src/tree/lib/libthr/thread/thr_stack.c#n136.
>>>>> Issued by __aeabi_uidiv (since not all armv7 processors support HW
>>>>> divide).
>>>>>
>>>>> Unfortunately, I'm not sure how to fix it. The compiler can emit
>>>>> __aeabi_<> in any place, and I'm not sure if it can resolve all the
>>>>> symbols used by rtld_eld and libthr beforehand.
>>>>>
>>>>>
>>>>> Michal
>>>>>
>>>>
>>>> In this case (but not for all _aeabi_ functions) we can avoid division
>>>> as long as page size is a power of 2.
>>>>
>>>> The function is
>>>>
>>>> static inline size_t
>>>> round_up(size_t size)
>>>> {
>>>> if (size % _thr_page_size != 0)
>>>> size = ((size / _thr_page_size) + 1) *
>>>> _thr_page_size;
>>>> return size;
>>>> }
>>>>
>>>> The body can be condensed to
>>>>
>>>> return (size + _thr_page_size - 1) & ~(_thr_page_size - 1);
>>>>
>>>> This is shorter in both lines of code and instruction bytes.
>>>
>>> Lets not allow this to be lost. Could anybody confirm that the patch
>>> below fixes the issue?
>>>
>>> commit d560f4f6690a48476565278fd07ca131bf4eeb3c
>>> Author: Konstantin Belousov <k...@freebsd.org>
>>> Date: Wed Jul 24 13:17:55 2024 +0300
>>>
>>> rtld: avoid division in __thr_map_stacks_exec()
>>> The function is called by rtld with the rtld bind lock write-locked,
>>> when fixing the stack permission during dso load. Not every ARMv7 CPU
>>> supports the div, which causes the recursive entry into rtld to resolve
>>> the __aeabi_uidiv symbol, causing self-lock.
>>> Workaround the problem by using roundup2() instead of open-coding less
>>> efficient formula.
>>> Diagnosed by: mmel
>>> Based on submission by: John F Carr <j...@mit.edu>
>>> Sponsored by: The FreeBSD Foundation
>>> MFC after: 1 week
>>>
> Just realized that it is wrong. Stack size is user-controlled and it does
> not need to be power of two.
Your change is correct. _thr_page_size is set to getpagesize(),
which is a power of 2. The call to roundup2 takes a user-provided
size and rounds it up to a multiple of the system page size.
I tested the change and it works. My change also works and
should compile to identical code. I forgot there was a standard
function to do the rounding.
> For final resolving of deadlocks, after a full day of digging, I'm very much
>> incline of adding -znow to the linker flags for libthr.so (and maybe also
>> for ld-elf.so). The runtime cost of resolving all symbols at startup is very
>> low. Direct pre-solving in _thr_rtld_init() is problematic for the _aeabi_*
>> symbols, since they don't have an official C prototypes, and some are not
>> compatible with C calling conventions.
> I do not like it. `-z now' changes (breaks) the ABI and makes some symbols
> not preemtible.
>
> In the worst case, we would need a call to the asm routine which causes the
> resolution of the _eabi_* symbols on arm.
>
It would also be possible to link libthr with libgcc.a and use a linker map
to hide the _eabi_ symbols.
