[PATCH] ELF: document some de-facto PT_* ABI quirks

[Alexey Dobriyan]

Turns out rules about PT_INTERP, PT_GNU_STACK and PT_GNU_PROPERTY
segment headers are slightly different.

Signed-off-by: Alexey Dobriyan <adobriyan@xxxxxxxxx>
---

 Documentation/ELF/ELF.rst |   28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

new file mode 100644
--- /dev/null
+++ b/Documentation/ELF/ELF.rst
@@ -0,0 +1,28 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+Definitions
+===========
+
+"First" program header is the one with the smallest offset in the file:
+e_phoff. "Last" program header is the one with the biggest offset:
+e_phoff + (e_phnum - 1) * sizeof(Elf_Phdr).
+
+PT_INTERP
+=========
+
+First PT_INTERP program header is used to locate the filename of ELF
+interpreter. Other PT_INTERP headers are ignored (since Linux 2.4.11).
+
+PT_GNU_STACK
+============
+
+Last PT_GNU_STACK program header defines userspace stack executability
+(since Linux 2.6.6). Other PT_GNU_STACK headers are ignored.
+
+PT_GNU_PROPERTY
+===============
+
+ELF interpreter's last PT_GNU_PROPERTY program header is used (since
+Linux 5.8). If interpreter doesn't have one, then the last PT_GNU_PROPERTY
+program header of an executable is used. Other PT_GNU_PROPERTY headers
+are ignored.