Re: [PATCH RFC 1/1] elf: align the mapping address of LOAD segments with p_align

[Rongwei Wang <rongwei.wang@linux.alibaba.com>]

On 12/9/21 7:52 AM, H.J. Lu wrote:
> On Tue, Dec 7, 2021 at 7:05 PM Rongwei Wang
> <rongwei.wang@linux.alibaba.com> wrote:
>>
>>
>>
>> On 12/8/21 10:33 AM, H.J. Lu wrote:
>>> On Tue, Dec 7, 2021 at 6:14 PM Rongwei Wang
>>> <rongwei.wang@linux.alibaba.com> wrote:
>>>>
>>>> Hi hjl
>>>>
>>>> On 12/6/21 10:48 PM, H.J. Lu wrote:
>>>>> On Fri, Dec 3, 2021 at 9:00 PM Rongwei Wang via Libc-alpha
>>>>> <libc-alpha@sourceware.org> wrote:
>>>>>>
>>>>>> Now, ld.so always map the LOAD segments and aligned by base
>>>>>> page size (e.g. 4k in x86 or 4k, 16k and 64k in arm64). And
>>>>>> this patch improve the scheme here. In this patch, ld.so
>>>>>> can align the mapping address of the first LOAD segment with
>>>>>> p_align when p_align is greater than the current base page
>>>>>> size.
>>>>>
>>>>> This is a bug fix.  Please open a glibc bug:
>>>>>
>>>>> https://sourceware.org/bugzilla/enter_bug.cgiOK
>>>>
>>>> And I requesting the account.
>>>>>
>>>>> with a testcase which should align variables to 2MB in the main
>>>> By the way, I have a question about whether we need to align each LOAD
>>>> segments? In our patch, we only fixed the mapping address for the first
>>>> LOAD segment.
>>>
>>> I think the first one should be sufficient.   You can verify it with a
>>> 2MB aligned variable in PIE:
>>>
>>> [hjl@gnu-cfl-2 tmp]$ cat x.c
>>> #include <stdio.h>
>>>
>>> int foo  __attribute__((aligned(0x200000))) = 1;
>>>
>>> int
>>> main ()
>>> {
>>>     printf ("foo: %p\n", &foo);
>>> }
>>> [hjl@gnu-cfl-2 tmp]$ gcc -no-pie x.c
>>> [hjl@gnu-cfl-2 tmp]$ ./a.out
>>> foo: 0x800000
>>> [hjl@gnu-cfl-2 tmp]$ gcc x.c -fpie -pie
>>> [hjl@gnu-cfl-2 tmp]$ ./a.out
>>> foo: 0x55c529afe000
>>> [hjl@gnu-cfl-2 tmp]$
>>
>> Learned it!
> 
> I opened:Thanks.
> 
> https://sourceware.org/bugzilla/show_bug.cgi?id=28676
Hi, I saw your report a kernel bug about PIE:

https://bugzilla.kernel.org/show_bug.cgi?id=215275

I remember a related fix patch to this bug is also included in our patchset:

https://lore.kernel.org/linux-mm/20211009092658.59665-4-rongwei.wang@linux.alibaba.com/

So, this issue is regarded as a bug by glibc, I can resend this patch
to kernel mail list and CC you.

Thanks.
> 
>> Thanks.
>>>
>>>>> program and a shared library.   Please include the testcase in
>>>>> your patch and mention the glibc bug in your commit message.
>>>>>
>>>>>> And this change makes code segments using huge pages become
>>>>>> simple and available.
>>>>>>
>>>>>> Signed-off-by: Rongwei Wang <rongwei.wang@linux.alibaba.com>
>>>>>> ---
>>>>>>     elf/dl-load.c         |  1 +
>>>>>>     elf/dl-map-segments.h | 54 +++++++++++++++++++++++++++++++++++++++++--
>>>>>>     include/link.h        |  3 +++
>>>>>>     3 files changed, 56 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/elf/dl-load.c b/elf/dl-load.c
>>>>>> index e39980fb19..136cfe2fa8 100644
>>>>>> --- a/elf/dl-load.c
>>>>>> +++ b/elf/dl-load.c
>>>>>> @@ -1154,6 +1154,7 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
>>>>>>              c->dataend = ph->p_vaddr + ph->p_filesz;
>>>>>>              c->allocend = ph->p_vaddr + ph->p_memsz;
>>>>>>              c->mapoff = ALIGN_DOWN (ph->p_offset, GLRO(dl_pagesize));
>>>>>> +          l->l_load_align = ph->p_align;
>>>>>>
>>>>>>              /* Determine whether there is a gap between the last segment
>>>>>>                 and this one.  */
>>>>>> diff --git a/elf/dl-map-segments.h b/elf/dl-map-segments.h
>>>>>> index ac9f09ab4c..ae03236045 100644
>>>>>> --- a/elf/dl-map-segments.h
>>>>>> +++ b/elf/dl-map-segments.h
>>>>>> @@ -18,6 +18,47 @@
>>>>>>
>>>>>>     #include <dl-load.h>
>>>>>>
>>>>>> +static __always_inline void *
>>>>>> +_dl_map_segments_align (const struct loadcmd *c,
>>>>>> +                   ElfW(Addr) mappref, int fd, size_t alignment,
>>>>>> +                   const size_t maplength)
>>>>>> +{
>>>>>> +       unsigned long map_start, map_start_align, map_end;
>>>>>> +       unsigned long maplen = (maplength >= alignment) ?
>>>>>> +                               (maplength + alignment) : (2 * alignment);
>>>>>> +
>>>>>> +       /* Allocate enough space to ensure that address aligned by
>>>>>> +           p_align is included. */
>>>>>> +       map_start = (ElfW(Addr)) __mmap ((void *) mappref, maplen,
>>>>>> +                                    PROT_NONE,
>>>>>> +                                    MAP_ANONYMOUS | MAP_PRIVATE,
>>>>>> +                                    -1, 0);
>>>>>> +       if (__glibc_unlikely ((void *) map_start == MAP_FAILED)) {
>>>>>> +               /* If mapping a aligned address failed, then ... */
>>>>>> +               map_start = (ElfW(Addr)) __mmap ((void *) mappref, maplength,
>>>>>> +                                    c->prot,
>>>>>> +                                    MAP_COPY|MAP_FILE,
>>>>>> +                                    fd, c->mapoff);
>>>>>> +
>>>>>> +               return (void *) map_start;
>>>>>> +       }
>>>>>> +       map_start_align = ALIGN_UP(map_start, alignment);
>>>>>> +       map_end = map_start_align + maplength;
>>>>>> +
>>>>>> +       /* Remember which part of the address space this object uses.  */
>>>>>> +       map_start_align = (ElfW(Addr)) __mmap ((void *) map_start_align, maplength,
>>>>>> +                                    c->prot,
>>>>>> +                                    MAP_COPY|MAP_FILE|MAP_FIXED,
>>>>>> +                                    fd, c->mapoff);
>>>>>> +       if (__glibc_unlikely ((void *) map_start_align == MAP_FAILED))
>>>>>> +               return MAP_FAILED;
>>>>>> +       if (map_start_align > map_start)
>>>>>> +               __munmap((void *)map_start, map_start_align - map_start);
>>>>>> +       __munmap((void *)map_end, map_start + maplen - map_end);
>>>>>> +
>>>>>> +       return (void *) map_start_align;
>>>>>> +}
>>>>>> +
>>>>>
>>>>> Please follow the glibc coding format.
>>>>>
>>>>>>     /* This implementation assumes (as does the corresponding implementation
>>>>>>        of _dl_unmap_segments, in dl-unmap-segments.h) that shared objects
>>>>>>        are always laid out with all segments contiguous (or with gaps
>>>>>> @@ -52,11 +93,20 @@ _dl_map_segments (struct link_map *l, int fd,
>>>>>>                                       c->mapstart & GLRO(dl_use_load_bias))
>>>>>>                - MAP_BASE_ADDR (l));
>>>>>>
>>>>>> -      /* Remember which part of the address space this object uses.  */
>>>>>> -      l->l_map_start = (ElfW(Addr)) __mmap ((void *) mappref, maplength,
>>>>>> +       /* During mapping, align the mapping address of the LOAD segments
>>>>>> +          according to own p_align. This helps OS map its code segment to
>>>>>> +          huge pages. */
>>>>>> +       if (l->l_load_align > GLRO(dl_pagesize)) {
>>>>>> +               l->l_map_start = (ElfW(Addr)) _dl_map_segments_align (c,
>>>>>> +                                            mappref, fd,
>>>>>> +                                            l->l_load_align, maplength);
>>>>>> +       } else {
>>>>>> +               /* Remember which part of the address space this object uses.  */
>>>>>> +               l->l_map_start = (ElfW(Addr)) __mmap ((void *) mappref, maplength,
>>>>>>                                                 c->prot,
>>>>>>                                                 MAP_COPY|MAP_FILE,
>>>>>>                                                 fd, c->mapoff);
>>>>>
>>>>> Please follow the glibc coding format.
>>>> OK
>>>>
>>>> Thanks.
>>>>>
>>>>>> +       }
>>>>>>           if (__glibc_unlikely ((void *) l->l_map_start == MAP_FAILED))
>>>>>>             return DL_MAP_SEGMENTS_ERROR_MAP_SEGMENT;
>>>>>>
>>>>>> diff --git a/include/link.h b/include/link.h
>>>>>> index aea268439c..fc6ce29fab 100644
>>>>>> --- a/include/link.h
>>>>>> +++ b/include/link.h
>>>>>> @@ -298,6 +298,9 @@ struct link_map
>>>>>>
>>>>>>         /* Thread-local storage related info.  */
>>>>>>
>>>>>> +    /* Alignment requirement of the LOAD block.  */
>>>>>> +    size_t l_load_align;
>>>>>> +
>>>>>>         /* Start of the initialization image.  */
>>>>>>         void *l_tls_initimage;
>>>>>>         /* Size of the initialization image.  */
>>>>>> --
>>>>>> 2.27.0
>>>>>>
>>>>>
>>>>> Thanks.
>>>>>
>>>
>>>
>>>
> 
> 
>