The quilt patch titled Subject: proc: give /proc/cmdline size has been removed from the -mm tree. Its filename was proc-give-proc-cmdline-size.patch This patch was dropped because it was merged into the mm-nonmm-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm ------------------------------------------------------ From: Alexey Dobriyan <adobriyan@xxxxxxxxx> Subject: proc: give /proc/cmdline size Date: Thu, 8 Sep 2022 21:21:54 +0300 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: 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. P.S.: I tried to scp /proc/cpuinfo today and got empty file but this is separate story. Link: https://lkml.kernel.org/r/YxoywlbM73JJN3r+@localhost.localdomain Signed-off-by: Alexey Dobriyan <adobriyan@xxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- fs/proc/cmdline.c | 6 +++++- include/linux/init.h | 1 + init/main.c | 7 +++++-- 3 files changed, 11 insertions(+), 3 deletions(-) --- a/fs/proc/cmdline.c~proc-give-proc-cmdline-size +++ a/fs/proc/cmdline.c @@ -3,6 +3,7 @@ #include <linux/init.h> #include <linux/proc_fs.h> #include <linux/seq_file.h> +#include "internal.h" static int cmdline_proc_show(struct seq_file *m, void *v) { @@ -13,7 +14,10 @@ static int cmdline_proc_show(struct seq_ static int __init proc_cmdline_init(void) { - proc_create_single("cmdline", 0, NULL, cmdline_proc_show); + struct proc_dir_entry *pde; + + pde = proc_create_single("cmdline", 0, NULL, cmdline_proc_show); + pde->size = saved_command_line_len + 1; return 0; } fs_initcall(proc_cmdline_init); --- a/include/linux/init.h~proc-give-proc-cmdline-size +++ a/include/linux/init.h @@ -143,6 +143,7 @@ struct file_system_type; extern int do_one_initcall(initcall_t fn); extern char __initdata boot_command_line[]; extern char *saved_command_line; +extern unsigned int saved_command_line_len; extern unsigned int reset_devices; /* used by init/main.c */ --- a/init/main.c~proc-give-proc-cmdline-size +++ a/init/main.c @@ -145,7 +145,8 @@ void (*__initdata late_time_init)(void); /* Untouched command line saved by arch-specific code. */ char __initdata boot_command_line[COMMAND_LINE_SIZE]; /* Untouched saved command line (eg. for /proc) */ -char *saved_command_line; +char *saved_command_line __ro_after_init; +unsigned int saved_command_line_len __ro_after_init; /* Command line for parameter parsing */ static char *static_command_line; /* Untouched extra command line */ @@ -667,6 +668,8 @@ static void __init setup_command_line(ch strcpy(saved_command_line + len, extra_init_args); } } + + saved_command_line_len = strlen(saved_command_line); } /* @@ -1379,7 +1382,7 @@ static void __init do_initcall_level(int static void __init do_initcalls(void) { int level; - size_t len = strlen(saved_command_line) + 1; + size_t len = saved_command_line_len + 1; char *command_line; command_line = kzalloc(len, GFP_KERNEL); _ Patches currently in -mm which might be from adobriyan@xxxxxxxxx are