On Thu, 8 Sep 2022 21:21:54 +0300 Alexey Dobriyan <adobriyan@xxxxxxxxx> wrote:
> Most /proc files don't have length (in fstat sense). This leads
> to inefficiencies when reading such files with APIs commonly found in
> modern programming languages. They open file, then fstat descriptor,
> get st_size == 0 and either assume file is empty or start reading
> without knowing target size.
>
> cat(1) does OK because it uses large enough buffer by default.
> But naive programs copy-pasted from SO aren't:
What is "SO"?
> let mut f = std::fs::File::open("/proc/cmdline").unwrap();
> let mut buf: Vec<u8> = Vec::new();
> f.read_to_end(&mut buf).unwrap();
>
> will result in
>
> openat(AT_FDCWD, "/proc/cmdline", O_RDONLY|O_CLOEXEC) = 3
> statx(0, NULL, AT_STATX_SYNC_AS_STAT, STATX_ALL, NULL) = -1 EFAULT (Bad address)
> statx(3, "", AT_STATX_SYNC_AS_STAT|AT_EMPTY_PATH, STATX_ALL, {stx_mask=STATX_BASIC_STATS|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0444, stx_size=0, ...}) = 0
> lseek(3, 0, SEEK_CUR) = 0
> read(3, "BOOT_IMAGE=(hd3,gpt2)/vmlinuz-5.", 32) = 32
> read(3, "19.6-100.fc35.x86_64 root=/dev/m", 32) = 32
> read(3, "apper/fedora_localhost--live-roo"..., 64) = 64
> read(3, "ocalhost--live-swap rd.lvm.lv=fe"..., 128) = 116
> read(3, "", 12)
>
> open/stat is OK, lseek looks silly but there are 3 unnecessary reads
> because Rust starts with 32 bytes per Vec<u8> and grows from there.
>
> In case of /proc/cmdline, the length is known precisely.
>
> Make variables readonly while I'm at it.
It seems arbitrary. Why does /proc/cmdline in particular get this
treatment?
