mmap causing bus error

mmap causing bus error

[Levo D]

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When I mmap a file it's possible to have a bus error and I attached some code to reproduce it

The reproduce steps are in main.c. I choose to use mmap because I noticed doing an `read` on a large file is a bit slow. It seems like the kernel will zero out everything then write the file contents to the buffer. mmap is significantly faster but risk corrupting data and having a bus error (also doesn't seem like zero copy).

Essentially what the program is is write to the contents of the file while main.c is using it which causes bad data OR it will TRUNC the file and cause a bus error + coredump. Are there ways to get a copy without being as 'slow' as `read` and doesn't have the downsides of mmap?

[-- Attachment #2: main.c --]
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/* Reproducable steps
gcc -g main.c -o app1
gcc -g app2.c -o app2
fn=/tmp/uB2kCDFlDoRA56gdeuvOVKbL1KHXxMhDXc3pFnhjbhc
dd if=/dev/zero of=$fn count=5120 bs=4096; ./app1 $fn 1 | ./app2 $fn
You'll see "Result was 72" instead of "Result was 0"
(app 2 modifies $fn so we'll need dd again)
dd if=/dev/zero of=$fn count=5120 bs=4096; ./app1 $fn 0 | ./app2 $fn
You'll see the desired "Result was 0"
In app2.c uncomment/comment line 13 and 12
gcc app2.c -o app2
Now this will have a bus error and "Result" line will not print
dd if=/dev/zero of=$fn count=5120 bs=4096; ./app1 $fn 1 | ./app2 $fn
read is unaffected
dd if=/dev/zero of=$fn count=5120 bs=4096; ./app1 $fn 0 | ./app2 $fn
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>
int main(int argc, char *argv[])
{
	//a.out filename 1
	if (argc != 3) {
		fprintf(stderr, "Bad args\n");
		return 1;
	}
	
	int fd = open(argv[1], O_RDONLY);
	struct stat s={0};
	fstat(fd, &s);
	char*p;
	if (argv[2][0] == '1')
		p = (char*)mmap(0, s.st_size, PROT_READ , MAP_PRIVATE, fd, 0);
	else {
		p = (char*)malloc(s.st_size);
		read(fd, p, s.st_size);
	}
	write(2, "Sleeping\n", 9);
	write(1, "Sleeping\n", 9); //Triggers the other app to write
	sleep(1);
	write(2, "Waking\n", 7);

	//read first byte of every 4k page
	int sum = 0;
	for (long i=0; i<s.st_size; i+=4096) {
		if (p[i] != 0) {
			int z=0;
		}
		sum += p[i];
	}
	fprintf(stderr, "Result was %d\n", sum);
	return 0;
}


[-- Attachment #3: app2.c --]
[-- Type: application/octet-stream, Size: 401 bytes --]

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
int main(int argc, char *argv[])
{
	if (argc != 2) {
		fprintf(stderr, "Bad args\n");
		return 1;
	}
	char buf[4096];
	read(0, buf, 4096);
	int fd = open(argv[1], O_WRONLY|O_CREAT);
	//int fd = open(argv[1], O_WRONLY|O_CREAT|O_TRUNC|O_CLOEXEC); //<-- This one causes a bus error
	write(fd, "Hello", 5);
	fprintf(stderr, "Written %d\n", fd);
}