Re: race between vfs_rename and do_linkat (mv and link)

[Xavier Roche <xavier.roche@algolia.com>]

On Wed, Feb 16, 2022 at 11:28:18AM +0100, Miklos Szeredi wrote:
> Something like this:
> diff --git a/fs/namei.c b/fs/namei.c
> index 3f1829b3ab5b..dd6908cee49d 100644

Tested-by: Xavier Roche <xavier.roche@algolia.com>

I confirm this completely fixes at least the specific race. Tested on a
unpatched and then patched 5.16.5, with the trivial bash test, and then
with a C++ torture test.

Before:
-------

$ time ./linkbug
Failed after 4 with No such file or directory

real	0m0,004s
user	0m0,000s
sys	0m0,004s

After:
------

(no error after ten minutes of running the program)

Torture test program:
---------------------

/* Linux rename vs. linkat race condition.
 * Rationale: both (1) moving a file to a target and (2) linking the target to a file in parallel leads to a race
 * on Linux kernel.
 * Sample file courtesy of Xavier Grand at Algolia
 * g++ -pthread linkbug.c -o linkbug
 */

#include <thread>
#include <unistd.h>
#include <assert.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <iostream>
#include <string.h>

static const char* producedDir = "/tmp";
static const char* producedFile = "/tmp/file.txt";
static const char* producedTmpFile = "/tmp/file.txt.tmp";
static const char* producedThreadDir = "/tmp/tmp";
static const char* producedThreadFile = "/tmp/file.txt.tmp.2";

bool createFile(const char* path)
{
    const int fdOut = open(path,
                           O_WRONLY | O_CREAT | O_TRUNC | O_EXCL | O_CLOEXEC,
                           S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH);
    assert(fdOut != -1);
    assert(write(fdOut, "Foo", 4) == 4);
    assert(close(fdOut) == 0);
    return true;
}

void func()
{
    int nbSuccess = 0;
    // Loop producedThread a hardlink of the file
    while (true) {
        if (link(producedFile, producedThreadFile) != 0) {
            std::cout << "Failed after " << nbSuccess << " with " << strerror(errno) << std::endl;
            exit(EXIT_FAILURE);
        } else {
            nbSuccess++;
        }
        assert(unlink(producedThreadFile) == 0);
    }
}

int main()
{
    // Setup env
    unlink(producedTmpFile);
    unlink(producedFile);
    unlink(producedThreadFile);
    createFile(producedFile);
    mkdir(producedThreadDir, 0777);

    // Async thread doing a hardlink and moving it
    std::thread t(func);
    // Loop creating a .tmp and moving it
    while (true) {
        assert(createFile(producedTmpFile));
        assert(rename(producedTmpFile, producedFile) == 0);
    }
    return 0;
}