[musl] [PATCH 0/3] use new SYS_fchmodat2 syscall to implement fchmodat with flags

["Gaël PORTAY" <gael.portay@rtone.fr>]

hello,

this patch serie adds the syscall SYS_fchmodat2 to implement fchmodat().
it has landed in linux v6.6.

the first patch remove the spurious flag argument from the syscall
SYS_fchmodat (linux lacks for that argument actually).

the second patch defines the syscall itself in headers.

the third patch uses the syscall SYS_fchmodat2 to implement fchmodat()
if flags are passed. the same way the syscall SYS_faccessat2 was used to
implement faccessat() with flags.

it wat tested locally on x86_64 (only), with a dummy test-fchmodat.c.

here is what strace says if no flags are passed:

	gportay@archlinux ~/src/musl $ strace ./lib/libc.so ./test-fchmodat . file 0664 0
	execve("./lib/libc.so", ["./lib/libc.so", "./test-fchmodat", ".", "file", "0664", "0"], 0x7ffc172ccc68 /* 34 vars */) = 0
	arch_prctl(ARCH_SET_FS, 0x765732cf7ba8) = 0
	set_tid_address(0x765732cf8018)         = 367709
	open("./test-fchmodat", O_RDONLY|O_LARGEFILE) = 3
	read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\260\20\0\0\0\0\0\0"..., 960) = 960
	mmap(NULL, 20480, PROT_READ, MAP_PRIVATE, 3, 0) = 0x765732c58000
	mmap(0x765732c59000, 4096, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED, 3, 0x1000) = 0x765732c59000
	mmap(0x765732c5a000, 4096, PROT_READ, MAP_PRIVATE|MAP_FIXED, 3, 0x2000) = 0x765732c5a000
	mmap(0x765732c5b000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED, 3, 0x2000) = 0x765732c5b000
	close(3)                                = 0
	brk(NULL)                               = 0x5555568a6000
	brk(0x5555568a8000)                     = 0x5555568a8000
	mmap(0x5555568a6000, 4096, PROT_NONE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x5555568a6000
	mprotect(0x765732cf4000, 4096, PROT_READ) = 0
	mprotect(0x765732c5b000, 4096, PROT_READ) = 0
	fchmodat(AT_FDCWD, "file", 0664)        = 0
	exit_group(0)                           = ?
	+++ exited with 0 +++

it uses the syscall fchmodat.

and what strace says if AT_SYMLINK_NOFOLLOW is passed:

	gportay@archlinux ~/src/musl $ strace ./lib/libc.so ./test-fchmodat . file 0664 0x100
	execve("./lib/libc.so", ["./lib/libc.so", "./test-fchmodat", ".", "file", "0664", "0x100"], 0x7ffcd7228278 /* 34 vars */) = 0
	arch_prctl(ARCH_SET_FS, 0x7318d8907ba8) = 0
	set_tid_address(0x7318d8908018)         = 367715
	open("./test-fchmodat", O_RDONLY|O_LARGEFILE) = 3
	read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\260\20\0\0\0\0\0\0"..., 960) = 960
	mmap(NULL, 20480, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7318d8868000
	mmap(0x7318d8869000, 4096, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED, 3, 0x1000) = 0x7318d8869000
	mmap(0x7318d886a000, 4096, PROT_READ, MAP_PRIVATE|MAP_FIXED, 3, 0x2000) = 0x7318d886a000
	mmap(0x7318d886b000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED, 3, 0x2000) = 0x7318d886b000
	close(3)                                = 0
	brk(NULL)                               = 0x55555563d000
	brk(0x55555563f000)                     = 0x55555563f000
	mmap(0x55555563d000, 4096, PROT_NONE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x55555563d000
	mprotect(0x7318d8904000, 4096, PROT_READ) = 0
	mprotect(0x7318d886b000, 4096, PROT_READ) = 0
	fchmodat2(AT_FDCWD, "file", 0664, AT_SYMLINK_NOFOLLOW) = 0
	exit_group(0)                           = ?
	+++ exited with 0 +++

it uses the syscall fchmodat2!

note: I fails to subscribe to mailing list; please make sure I receive a
copy of any response, I am in cc; sorry for the inconvenience. I hope I
did everything correctly.

regards,
Gaël PORTAY (3):
  remove flag argument from fchmodat syscall
  bits/syscall.h: add __NR_fchmodat2 from linux v6.6
  use new SYS_fchmodat2 syscall to implement fchmodat with flags

 arch/arm/bits/syscall.h.in        | 1 +
 arch/i386/bits/syscall.h.in       | 1 +
 arch/m68k/bits/syscall.h.in       | 1 +
 arch/microblaze/bits/syscall.h.in | 1 +
 arch/mips/bits/syscall.h.in       | 1 +
 arch/mips64/bits/syscall.h.in     | 1 +
 arch/mipsn32/bits/syscall.h.in    | 1 +
 arch/or1k/bits/syscall.h.in       | 1 +
 arch/powerpc/bits/syscall.h.in    | 1 +
 arch/powerpc64/bits/syscall.h.in  | 1 +
 arch/riscv64/bits/syscall.h.in    | 1 +
 arch/s390x/bits/syscall.h.in      | 1 +
 arch/sh/bits/syscall.h.in         | 1 +
 arch/x32/bits/syscall.h.in        | 1 +
 arch/x86_64/bits/syscall.h.in     | 1 +
 src/stat/fchmodat.c               | 7 ++++++-
 16 files changed, 21 insertions(+), 1 deletion(-)

-- 
2.43.0