[Bug 220569] New: [truncate(2) man page] after extending file extra bytes are not always zero

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=220569

            Bug ID: 220569
           Summary: [truncate(2) man page] after extending file extra
                    bytes are not always zero
           Product: Documentation
           Version: unspecified
          Hardware: AMD
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P3
         Component: man-pages
          Assignee: documentation_man-pages@kernel-bugs.osdl.org
          Reporter: alanas.00+k@mail.ru
        Regression: No

truncate(2) man page has this sentence:

If the file previously was shorter, it is extended, and the extended part reads
as null bytes ('\0').

but it's not always true

how to cause truncate to extend file with non-zero bytes (run all commands as
root):

1. change working directory to ramfs mount (tmpfs same result):

mkdir r
mount -t ramfs asdf r
cd r

2. make create_weird_file.c file with content:

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>
int main(int argc,char**argv){
        if(argc!=2){
                puts("usage: create_weird_file /path/to/file");
                return 1;
        }
        int fd=open(argv[1],O_RDWR|O_CLOEXEC|O_CREAT|O_EXCL,0600);
        if(fd<0){
                printf("open error: %#m\n");
                return 1;
        }
        if(ftruncate(fd,1)){
                printf("ftruncate error: %#m\n");
                return 1;
        }
        void*mm=mmap(NULL,4,PROT_READ|PROT_WRITE,MAP_SHARED,fd,0);
        if(mm==MAP_FAILED){
                printf("mmap error: %#m\n");
                return 1;
        }
        *(int*)mm=1717859169;
}

3. make truncate_4.c file with content:

#include <stdio.h>
#include <unistd.h>
int main(int argc,char**argv){
        if(argc!=2){
                puts("usage: truncate_4 /path/to/file");
                return 1;
        }
        if(truncate(argv[1],4)){
                printf("truncate error: %#m\n");
                return 1;
        }
}

4. compile create_weird_file.c:

gcc -o create_weird_file create_weird_file.c

5. compile truncate_4.c

gcc -o truncate_4 truncate_4.c

6. create 1 byte file f:

./create_weird_file f

7. see file content and size:

xxd f

output for me is:

00000000: 61                                       a

8. truncate file f to 4 bytes:

./truncate_4 f

9. see file content and size:

xxd f

output for me is:

00000000: 6173 6466                                asdf

extra 3 bytes of file are "sdf" instead of "\0\0\0"

tested on 2 operating systems:
kde neon unstable, uname -r -v -m -p -i -o: 6.14.0-29-generic
#29~24.04.1-Ubuntu SMP PREEMPT_DYNAMIC Thu Aug 14 16:52:50 UTC 2 x86_64 x86_64
x86_64 GNU/Linux
opensuse tumbleweed, uname -r -v -m -p -i -o: 6.16.5-1-default #1 SMP
PREEMPT_DYNAMIC Thu Sep  4 15:51:43 UTC 2025 (642f24d) x86_64 x86_64 x86_64
GNU/Linux

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