linux-kernel - Re: [PATCH] proc: give /proc/cmdline size

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Date:   Thu, 8 Sep 2022 13:45:46 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] proc: give /proc/cmdline size

On Thu, 8 Sep 2022 21:21:54 +0300 Alexey Dobriyan <adobriyan@...il.com> 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?