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authorAlexey Dobriyan <adobriyan@gmail.com>2022-09-08 21:21:54 +0300
committerAndrew Morton <akpm@linux-foundation.org>2022-11-18 13:55:07 -0800
commit941baf6febaa88c057192084ca280d976c7c7239 (patch)
treeb38ce270a9f8c6c5f6f2f157a3b174d41d464687 /fs/proc
parent9b71f79f6e349251e529ef25f2d3bf8aa6168d26 (diff)
proc: give /proc/cmdline size
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@gmail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Diffstat (limited to 'fs/proc')
-rw-r--r--fs/proc/cmdline.c6
1 files changed, 5 insertions, 1 deletions
diff --git a/fs/proc/cmdline.c b/fs/proc/cmdline.c
index fa762c5fbcb2..91fe1597af7b 100644
--- a/fs/proc/cmdline.c
+++ b/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_file *m, void *v)
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);