WASMFS write() on an `O_RDONLY` fd silently succeeds

[Ryan2026R]

Version of emscripten/emsdk:

5.0.6

POSIX: write() on a file descriptor opened O_RDONLY should return -1. It seems that the default JS-FS does this correctly; but under WASMFS (-sWASMFS=1) the write succeeds.

Failing command line in full:

$ emcc -O0 -Wall -sWASMFS=1 write_rdonly.c -o a.out.js
$ node a.out.js
write(RDONLY fd): ret=1 errno=0

The same program on native Linux prints write(RDONLY fd): ret=-1 errno=9.

Example code snippet:

#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>
#include <stdio.h>
#include <errno.h>

int main(void) {
    mkdir("/tmp/xsv_test", 0755);
    int fd = open("/tmp/xsv_test/f.txt", O_WRONLY | O_CREAT | O_TRUNC, 0644);
    write(fd, "x", 1); close(fd);

    fd = open("/tmp/xsv_test/f.txt", O_RDONLY);
    errno = 0;
    ssize_t r = write(fd, "y", 1);
    printf("write(RDONLY fd): ret=%zd errno=%d\n", r, errno);
    close(fd);
    return (r < 0) ? 0 : 1;
}