[PATCH resend 0/2] btrfs: fix races between exec and defrag

* [PATCH resend 0/2] btrfs: fix races between exec and defrag
@ 2018-07-17 22:05 Adam Borowski
  2018-07-17 22:08 ` [PATCH resend 1/2] btrfs: allow defrag on a file opened ro that has rw permissions Adam Borowski
  0 siblings, 1 reply; 6+ messages in thread
From: Adam Borowski @ 2018-07-17 22:05 UTC (permalink / raw)
  To: linux-btrfs, David Sterba, Mark Fasheh

[-- Attachment #1: Type: text/plain, Size: 949 bytes --]

Hi!
Here's a ping for a patch to fix ETXTBSY races between defrag and exec, just
like the dedupe counterpart.  Unlike that one which is shared to multiple
filesystems and thus lives in Al Viro's land, it is btrfs only.

Attached: a simple tool to fragment a file, by ten O_SYNC rewrites of length
1 at random positions; racey vs concurrent writes or execs but shouldn't
damage the file otherwise.

Also attached: a preliminary patch for -progs; it yet lacks a check for the
kernel version, but to add such a check we'd need to know which kernels
actually permit ro defrag for non-root.

No man page patch -- there's no man page to be patched...


Meow!
-- 
// If you believe in so-called "intellectual property", please immediately
// cease using counterfeit alphabets.  Instead, contact the nearest temple
// of Amon, whose priests will provide you with scribal services for all
// your writing needs, for Reasonable And Non-Discriminatory prices.

[-- Attachment #2: fragme.c --]
[-- Type: text/x-csrc, Size: 1363 bytes --]

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdarg.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <unistd.h>
#include <sys/syscall.h>

static void die(const char *txt, ...) __attribute__((format (printf, 1, 2)));
static void die(const char *txt, ...)
{
    fprintf(stderr, "fragme: ");

    va_list ap;
    va_start(ap, txt);
    vfprintf(stderr, txt, ap);
    va_end(ap);

    exit(1);
}

static uint64_t rnd(uint64_t max)
{
    __uint128_t r;
    if (syscall(SYS_getrandom, &r, sizeof(r), 0)==-1)
        die("getrandom(): %m\n");
    return r%max;
}

int main(int argc, char **argv)
{
    if (argc!=2)
        die("Usage: fragme <file>\n");

    int fd = open(argv[1], O_RDWR|O_SYNC);
    if (fd == -1)
        die("open(\"%s\"): %m\n", argv[1]);
    off_t size = lseek(fd, 0, SEEK_END);
    if (size == -1)
        die("lseek(SEEK_END): %m\n");

    for (int i=0; i<10; ++i)
    {
        off_t off = rnd(size);
        char b;
        if (lseek(fd, off, SEEK_SET) != off)
            die("lseek for read: %m\n");
        if (read(fd, &b, 1) != 1)
            die("read(%lu): %m\n", off);
        if (lseek(fd, off, SEEK_SET) != off)
            die("lseek for write: %m\n");
        if (write(fd, &b, 1) != 1)
            die("write: %m\n");
    }

    return 0;
}

[-- Attachment #3: 0001-defrag-open-files-RO.patch --]
[-- Type: text/x-diff, Size: 1140 bytes --]

>From d040af09adb03daadbba4336700f40425a860320 Mon Sep 17 00:00:00 2001
From: Adam Borowski <kilobyte@angband.pl>
Date: Tue, 28 Nov 2017 01:00:21 +0100
Subject: [PATCH] defrag: open files RO

NOT FOR MERGING -- requires kernel versioning

Fixes EXTXBSY races.

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
---
 cmds-filesystem.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/cmds-filesystem.c b/cmds-filesystem.c
index 30a50bf5..7eb6b7bb 100644
--- a/cmds-filesystem.c
+++ b/cmds-filesystem.c
@@ -876,7 +876,7 @@ static int defrag_callback(const char *fpath, const struct stat *sb,
 	if ((typeflag == FTW_F) && S_ISREG(sb->st_mode)) {
 		if (defrag_global_verbose)
 			printf("%s\n", fpath);
-		fd = open(fpath, O_RDWR);
+		fd = open(fpath, O_RDONLY);
 		if (fd < 0) {
 			goto error;
 		}
@@ -1012,7 +1012,7 @@ static int cmd_filesystem_defrag(int argc, char **argv)
 		int defrag_err = 0;
 
 		dirstream = NULL;
-		fd = open_file_or_dir(argv[i], &dirstream);
+		fd = open_file_or_dir3(argv[i], &dirstream, O_RDONLY);
 		if (fd < 0) {
 			error("cannot open %s: %m", argv[i]);
 			ret = -errno;
-- 
2.18.0


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