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Date: Mon, 5 Aug 2024 09:22:08 +0200
From: Christian Brauner <brauner@kernel.org>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, 
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH] fix bitmap corruption on close_range() with
 CLOSE_RANGE_UNSHARE
Message-ID: <20240805-modisch-anstreben-dc6f70ad6d3e@brauner>
References: <20240803225054.GY5334@ZenIV>
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> Reproducer follows:
> 
> #define __GNU_SOURCE
> #include <linux/close_range.h>
> #include <unistd.h>
> #include <fcntl.h>
> #include <signal.h>
> #include <sched.h>
> #include <stdio.h>
> #include <stdbool.h>
> #include <sys/mman.h>
>     
> void is_open(int fd)
> {
> 	printf("#%d is %s\n", fd,
> 		fcntl(fd, F_GETFD) >= 0 ? "opened" : "not opened");
> }
>     
> int child(void *unused)
> {
> 	while(1) {
> 	}
> 	return 0;
> }
>     
> int main(void)
> {
> 	char *stack;
> 	pid_t pid;
> 
> 	stack = mmap(NULL, 1024*1024, PROT_READ | PROT_WRITE,
> 		     MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
> 	if (stack == MAP_FAILED) {
> 		perror("mmap");
> 		return -1;
> 	}
> 
> 	pid = clone(child, stack + 1024*1024, CLONE_FILES | SIGCHLD, NULL);
> 	if (pid == -1) {
> 		perror("clone");
> 		return -1;
> 	}
> 	for (int i = 2; i < 128; i++)
> 	    dup2(0, i);
> 	close_range(64, ~0U, CLOSE_RANGE_UNSHARE);
> 
> 	is_open(64);
> 	printf("dup(0) => %d, expected 64\n", dup(0));
> 
> 	kill(pid, 9);
> 	return 0;

Could you please add that reproducer to
tools/testing/selftests/core/close_range_test.c

TEST(close_range_bitmap_corruption)
{
}

Really, it doesn't have to be pretty but these repros in there really
have been helpful finding such corruptions when run with a proper k*san
config.