[PATCH v3] ELF: document some de-facto PT_* ABI quirks

[Alexey Dobriyan <adobriyan@gmail.com>]

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

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

	v3: move to Documentation/userspace-api/
	v2: integrate into documentation build system

 Documentation/userspace-api/ELF.rst   |   34 ++++++++++++++++++++++++++++++++++
 Documentation/userspace-api/index.rst |    1 +
 2 files changed, 35 insertions(+)

new file mode 100644
--- /dev/null
+++ b/Documentation/userspace-api/ELF.rst
@@ -0,0 +1,34 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=================================
+Linux-specific ELF idiosyncrasies
+=================================
+
+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 in the file:
+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.
--- a/Documentation/userspace-api/index.rst
+++ b/Documentation/userspace-api/index.rst
@@ -23,6 +23,7 @@ place where this information is gathered.
    spec_ctrl
    accelerators/ocxl
    ebpf/index
+   ELF
    ioctl/index
    iommu
    iommufd