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Re: RFC: ABI support for special memory area
On Fri, Mar 31, 2017 at 6:44 AM, Suprateeka R Hegde
> On 30-Mar-2017 10:10 PM, H.J. Lu wrote:
>>> However, I am just thinking that your earlier approach --
>>> __gnu_mbind_setup -- is better when shared libraries with GNU_MBIND
>>> segments are dlopen'ed. They dont have to iterate all over again to
>>> reach their PHDR. Or what is the recommendation for such dlopen'ed
>> It is true that dl_iterate_phdr is called by every shared object, dlopened or
>> not, to locate its own PHDR.
> Lets put a one liner on best practices or guideline kind of. You have
> already made it clear in the example code. I am just thinking of putting
> them in words too.
> Lets say something like, each load module is expected to process only
> its special memory segments. To mean that shlibs/exe need not do any
> book-keeping to avoid multiple executions of the special memory setup
> for the same load module.
>>> And this dl_iterate_phdr(3) not being part of any standards, may change
>>> in a totally incompatible way in the future.
>> dl_iterate_phdr isn't in any standard. But it is in glibc. Given that my
>> proposal is a GNU extension, it isn't a major issue. Working with
>> existing glibc is a big plus.
> Awesome. Looks great. Thanks a lot for the new approach.
Here is the updated proposal.
ABI support for special memory area
To section attributes, add
#define SHF_GNU_MBIND 0x00100000
for sections used to place data or text into a special memory area.
The section names should start with ".mbind" so that they won't be
grouped together with normal sections by link editor. The sh_info
field indicates the special memory type. SHF_GNU_MBIND is only
applicable to SHF_ALLOC sections.
The following memory types in the sh_info field are defined:
/* The highest bandwidth memory. */
#define GNU_MBIND_HBW 0
To the "Program Header" section, add an inclusive range of segment types
for GNU_MBIND segments:
#define PT_GNU_MBIND_NUM 4096
#define PT_GNU_MBIND_LO (PT_LOOS + 0x474e555)
#define PT_GNU_MBIND_HI (PT_GNU_MBIND_LO + PT_GNU_MBIND_NUM - 1)
The array element specifies the location and size of a special memory area.
Each GNU_MBIND segment contains one GNU_MBIND section and the segment
type is PT_GNU_MBIND_LO plus the sh_info value. If the sh_info value is
greater than PT_GNU_MBIND_NUM, no GNU_MBIND segment will be created. Each
GNU_MBIND segment must be aligned at page boundary. The interpretation of
the special memory area information is implementation-dependent.
Implementations may ignore GNU_MBIND segment.
Each load module is expected to process only its special memory segments.
There is no need for executable and shared objects to do any book-keeping
to avoid multiple executions of the special memory setup for the same
dl_iterate_phdr in the the GNU C library:
int dl_iterate_phdr (int (*callback) (struct dl_phdr_info *info,
size_t size, void *data),
is called via the .init_array section to process GNU_MBIND segments in
executable and shared objects:
callback (struct dl_phdr_info *info, size_t size, void *data)
Compute the load address of the current module.
if info->dlpi_addr == the load address of the current module
check ELF program headers and process GNU_MBIND segments
dl_iterate_phdr (callback, NULL);
static void (*init_array) (void)
__attribute__ ((section (".init_array"), used))