[PATCH v2 00/22] nolibc: usability improvements (errno, environ, auxv)

Willy Tarreau
2023-01-10T07:25:11Z

Hello Paul,

this 3rd series aims at generally improving usability and maintainance
of nolibc. It needs to be applied on top of the s390 series:

https://lore.kernel.org/lkml/20230109080910.26594-1-w@1wt.eu/

- first, I've encountered remaining problems related to section
reordering happening at certain optimization levels and that is also
arch-dependent. We were fortunate they didn't appear in rcutorture,
but I could reproduce them with ARM in Thumb mode at -O0. The problem
is that our out-of-block asm() statement changes the current section
to ".text" without the compiler knowing. Thus the compiler may believe
it's still in .bss and emit variables immediately after (e.g. errno),
which end up in the wrong section. Switching the section at the end
to .bss doesn't work either because at -Os I was seeing sys_brk()
placed immediately after and crashing as .bss is not executable. The
only safe solution is to turn _start to real functions. This was
tested on all archs at -O0,-O1,-O2,-O3,-Os and all of them now work.

- second, thumb mode support was in complete on ARM. Only thumb-2 was
supported and depending on how the toolchain is configured, passing
"-mthumb" would result in thumb-1 (if armv4/5 was the default) or
thumb-2 (when armv7 was the default). I discovered this when first
trying the toolchains from kernel.org because mine works in v7 by
default, hence thumb-2. The change only replaces a few instructions
that are not available in thumb-1 by their compatible equivalent. In
addition, thumb cannot be used at -O0 or with frame pointers in general
because register r7 is the frame pointer there, and cannot be assigned
by the compiler. That's bad because r7 carries the syscall number. Now
when thumb is detected, we simply use a slightly larger setup code
which uses r6 and swaps it with r7 when performing the call.

- third, the definitions of the (possibly wrong) arch-specific O_* values
were dropped in favor of those coming from asm/fcntl.h. Not only these
ones are correct, but doing so will avoid build redefinition warnings
should the file be included for whatever other reason.

- the errno, environ and the auxiliary vector were really a pain to
use. errno was declared as a static variable, showing a different
one to each build unit. environ had to be declared by the application,
and the auxv had to be both declared and found by the application if
needed. All three of them have now been declared as weak symbols,
and environ and _auxv are setup during startup, so that only one
instance of them exists across the whole binary, and that code
currently declaring them continues to work. This now means that code
not using them will not optimize them away anymore, but let's face
it, errno was always used and no relevant application manages to get
rid of .bss, so the amount of extra space is really just 8-16 bytes
total for a much better simplicity for the user.

- getauxval() and getpagesize() were added by Ammar Faizi (along with
the associated selftests).

This was tested on arm64/armv5/armv7/thumb1/thumb2/i386/x86_64/mips/riscv
and s390 at all optimization levels. I could also verify that my original
preinit code continues to build and work fine, so please consider queuing
it.

Thank you!
Willy

Changes since v1:
- added missing s-o-b on the last 3 patches

Ammar Faizi (3):
nolibc/stdlib: Implement `getauxval(3)` function
nolibc/sys: Implement `getpagesize(2)` function
selftests/nolibc: Add `getpagesize(2)` selftest

Sven Schnelle (2):
tools/nolibc: export environ as a weak symbol on s390
tools/nolibc: add auxiliary vector retrieval for s390

Willy Tarreau (17):
tools/nolibc: make compiler and assembler agree on the section around
_start
tools/nolibc: enable support for thumb1 mode for ARM
tools/nolibc: support thumb mode with frame pointers on ARM
tools/nolibc: remove local definitions of O_* flags for open/fcntl
tools/nolibc: make errno a weak symbol instead of a static one
tools/nolibc: export environ as a weak symbol on x86_64
tools/nolibc: export environ as a weak symbol on i386
tools/nolibc: export environ as a weak symbol on arm64
tools/nolibc: export environ as a weak symbol on arm
tools/nolibc: export environ as a weak symbol on mips
tools/nolibc: export environ as a weak symbol on riscv
tools/nolibc: add auxiliary vector retrieval for i386
tools/nolibc: add auxiliary vector retrieval for x86_64
tools/nolibc: add auxiliary vector retrieval for arm64
tools/nolibc: add auxiliary vector retrieval for arm
tools/nolibc: add auxiliary vector retrieval for riscv
tools/nolibc: add auxiliary vector retrieval for mips

tools/include/nolibc/arch-aarch64.h | 52 +++----
tools/include/nolibc/arch-arm.h | 138 ++++++++++++-------
tools/include/nolibc/arch-i386.h | 60 ++++----
tools/include/nolibc/arch-mips.h | 79 ++++++-----
tools/include/nolibc/arch-riscv.h | 62 +++++----
tools/include/nolibc/arch-s390.h | 70 +++++-----
tools/include/nolibc/arch-x86_64.h | 52 +++----
tools/include/nolibc/errno.h | 4 +-
tools/include/nolibc/stdlib.h | 27 ++++
tools/include/nolibc/sys.h | 22 +++
tools/testing/selftests/nolibc/nolibc-test.c | 30 ++++
11 files changed, 363 insertions(+), 233 deletions(-)

--
2.17.5