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

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

On 12/8/21 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!

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.
>>>
> 
> 
>