mmap implementation for ANON mapping with desired address

[Norman Feske]

Hello Alexander,

On 21.09.20 23:33, Alexander Tormasov via users wrote:
> can you give me an example how I can obtain this Kernel::_heap?
> 
> initially I try something like this (from libc)
> 
> /* get Kernel::_heap from Libc */
> Genode::Allocator &alloc = reinterpret_cast<Libc::Env_implementation &>(env).vfs_env().alloc();
> 
> /* and use it as aux data storage */
> vm_reg.construct(env, reinterpret_cast<Genode::Heap &>(alloc));
> 
> but obvious that this will not work normally because (again) metadata space can’t be taken from this «main» process heap (this is the same address space for component which can clash with main program allocation).
> 
> I suspect that if I make inside my component something like
>     Heap _heap { _env.ram(), _env.rm() };
> 
> then again I will hit the same problem of clash of local virtual address with this metadata storage?
> How to «hide» this data from component programs?
> Or I can’t hide and need artificially pre-alloc area for such data and make separate heap based on this?

the libc already contains such an internal 'Heap' object. It is created
at [1] and passed to the constructor of the 'Libc::Kernel' [2]. The
Libc::Kernel keeps a reference to this 'Heap' instance [3]. When I spoke
of 'Kernel::_heap', I was referring to this instance.

Just below [1], you can see that the heap reference is passed to the
'Libc::init_fd_alloc' function. This way, the compilation unit
fd_alloc.cc [4] gets hold of this 'Kernel::Heap' instance and can
thereby use it for the allocation of libc-internal meta data, in this
case 'File_descriptor' objects.

The 'file_operations.cc' compilation unit that hosts the 'mmap'
implementation already has a 'Libc::init_file_operations' function but
the function does not take an 'Allocator &' as argument. In order to
allow this compilation unit to use the 'Kernel::_heap', you'll have to
extend this function with an 'Allocator &' argument (you may take
fd_alloc.cc as blueprint), and adjust the call at [5] accordingly
(passing '_heap' as second argument).

[1]
https://github.com/genodelabs/genode/blob/master/repos/libports/src/lib/libc/component.cc#L40
[2]
https://github.com/genodelabs/genode/blob/master/repos/libports/src/lib/libc/component.cc#L49
[3]
https://github.com/genodelabs/genode/blob/master/repos/libports/src/lib/libc/internal/kernel.h#L129
[4]
https://github.com/genodelabs/genode/blob/master/repos/libports/src/lib/libc/fd_alloc.cc
[5]
https://github.com/genodelabs/genode/blob/master/repos/libports/src/lib/libc/kernel.cc#L477

Cheers
Norman

-- 
Dr.-Ing. Norman Feske
Genode Labs

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