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SubjectRe: race between vfs_rename and do_linkat (mv and link)
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;
}

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