Hi,

This is the v1 of RFC to add dynamic memory allocator support for
nolibc.


## Background

The need to allocate memory dynamically has become a requirement for
the C programming language. Mainly it happens when the allocation size
is determined at runtime. Many other use cases also do it when the
object's lifetime is long-lived and needs to be recycled at runtime.

Currently, the nolibc header doesn't support such a type of allocation.
This series adds it.


## Implementation

Add basic functions to manage dynamic memory allocation:
  - malloc()
  - calloc()
  - realloc()
  - free()

The allocator uses mmap() syscall to allocate the memory and uses
munmap() syscall to free the allocated memory.

The metadata to keep track the length for munmap-ing is simply
defined as a struct below:
```
struct nolibc_heap {
        size_t  len;
        char    user_p[] __attribute__((__aligned__));
};
```
malloc(), realloc() and calloc() return a pointer to `user_p`.


## Add my_syscall6() support for x86 32-bit.

mmap() needs 6 arguments to work with. Not all architectures that
nolibc supports have the my_syscall6() wrapper. This series also
adds my_syscall6() wrapper support for i386.

Notes:

Both Clang and GCC cannot use %ebp in the clobber list and in the "r"
constraint without using -fomit-frame-pointer. To make it always
available for any kind of compilation, the below workaround is
implemented.

For clang (the Assembly statement can't clobber %ebp):
  1) Save the %ebp value to the redzone area -4(%esp).
  2) Load the 6-th argument from memory to %ebp.
  3) Subtract the %esp by 4.
  4) Do the syscall (int $0x80).
  5) Pop %ebp.

For GCC, fortunately it has a #pragma that can force a specific function
to be compiled with -fomit-frame-pointer, so it can use "r"(var) where
var is a variable bound to %ebp.


## Limitation

Currently, for mips and arm arch cannot use these dynamic memory allocator
functions because they're missing the my_syscall6() macro.

[ammarfaizi2: I would love to add the support for them too, but I don't
 have the hardware to play with MIPS and ARM. ]


## Test

The following simple program can be used to test this series:

  https://gist.github.com/ammarfaizi2/db0af6aa0b95a0c7478bce64e349f021


## Patchset Summary

1) Patch 1 is a fix for the System V ABI document link.

2) Patch 2 is a fix to support compile with clang.

3) Patch 3 adds my_syscall6() implementation for i386.

4) Patch 4 adds mmap() and munmap() functions.

5) Patch 5 adds malloc(), calloc(), realloc() and free().

6) Patch 6 adds strdup() and strndup().


Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
---
Ammar Faizi (6):
  tools/nolibc: x86-64: Update System V ABI document link
  tools/nolibc: Make the entry point not weak for clang
  tools/nolibc: i386: Implement syscall with 6 arguments
  tools/nolibc/sys: Implement `mmap()` and `munmap()`
  tools/nolibc/stdlib: Implement `malloc()`, `calloc()`, `realloc()` and `free()`
  tools/include/string: Implement `strdup()` and `strndup()`

 tools/include/nolibc/arch-aarch64.h |  2 +
 tools/include/nolibc/arch-arm.h     |  2 +
 tools/include/nolibc/arch-i386.h    | 66 ++++++++++++++++++++++++
 tools/include/nolibc/arch-mips.h    |  2 +
 tools/include/nolibc/arch-riscv.h   |  2 +
 tools/include/nolibc/arch-x86_64.h  |  4 +-
 tools/include/nolibc/stdlib.h       | 79 +++++++++++++++++++++++++++++
 tools/include/nolibc/string.h       | 68 +++++++++++++++++++++++++
 tools/include/nolibc/sys.h          | 62 ++++++++++++++++++++++
 9 files changed, 286 insertions(+), 1 deletion(-)


base-commit: fda0d5d1b79d8b7032be3d7720a481a9fde91baf
-- 
Ammar Faizi