Debugging a system call from FUSE

I'm writing a FUSE filesystem that does some mapping through sqlite, then passes the calls through to the underlying filesystem (somewhat of an expansion on bbfs). It started giving me trouble when I tried to start making files. When I call mknod, it returns with ERANGE. Here's the tail of an strace (filesystem is mounted on test/):

$ ./p4fs test/
$ strace touch test/kilo 2> logs
$ cat logs
...
fstat(3, {st_mode=S_IFREG|0644, st_size=56467024, ...}) = 0
mmap(NULL, 56467024, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fbf006bf000
close(3)                                = 0
close(0)                                = 0
open("test/kilo", O_WRONLY|O_CREAT|O_NOCTTY|O_NONBLOCK, 0666) = -1 ERANGE (Numerical result out of range)
futimesat(AT_FDCWD, "test/kilo", NULL)  = 0
close(1)                                = 0
exit_group(0)                           = ?

and here's the relevant section from my internal logging:

getattr: database opened
getattr: requesting attr for /kilo
db_getrowid: statement executed: finding rowid of /kilo
db_getrowid: mapped /kilo to rowid 0
getattr: does not exist: /kilo
mknod: database opened
mknod: statement executed: checking for existing path
mknod: calling db_mkdentry(db, /kilo, 100644, 0, 0)
db_mkdentry: parent is /
db_getrowid: statement executed: finding rowid of /
db_getrowid: mapped / to rowid 1
db_mkdentry: statement executed: creating dentry /kilo
db_getrowid: statement executed: finding rowid of /kilo
db_getrowid: mapped /kilo to rowid 3
p4fs: calling system mknod(3, 100644, 0)
p4fs: got errno 13

I'm looking for (1) the solution to this immediate problem and (2) a good way to debug FUSE in general. I have a sneaking suspicion that the ERANGE is coming from strtol(), but I don't know how to check. I wish I could make gdb pop up when it hits the callback...

Thanks!

EDIT: Oh, here's the source for my mknod() function:

static int p4_mknod(const char *path, mode_t mode, dev_t dev) {
        sqlite3 *db;
        sqlite3_stmt *statement;
        char query[MAX_QUERY_LENGTH];
        int rc;
        int return_value;
        int path_exists = -1;

开发者_运维问答        OPEN_LOG("mknod")
        OPEN_DB(db_path, db)

        /* check for existing filename */
        sprintf(query,
                "SELECT COUNT(*) FROM dentry "
                "WHERE name = '%s'",
                path);
        sqlite3_prepare(db,
                query,
                -1,
                &statement,
                NULL);
        rc = sqlite3_step(statement);
        SQLITE3_ERRCHK("checking for existing path")
        path_exists = sqlite3_column_int(statement, 0);
        sqlite3_finalize(statement);

        if (path_exists <= 0) {
                int physical_rowid;
                char physical_name[MAX_QUERY_LENGTH];

                /* path is not already in db */
                syslog(LOG_DEBUG, "calling db_mkdentry(db, %s, %o, 0, 0)",
                        path, mode);
                db_mkdentry(db, (char *) path, mode, 0, 0);

                /* make the actual file */
                physical_rowid = db_getrowid(db, (char *) path);
                sprintf(physical_name, "%i", physical_rowid);
                syslog(LOG_DEBUG, "calling system mknod(%s, %o, %li)",
                        physical_name, mode, dev);
                return_value = mknod(physical_name, mode, dev);
        } else {
                syslog(LOG_INFO, "called on existing path");
                return_value = -EEXIST;
        }

        syslog(LOG_DEBUG, "errno %i", errno);

        return errno;
}

A few pieces of advice:

• Do not use sprintf and friends to build SQL statements for sqlite3. It is recommended that you use hosted parameters in your statement and bind values to them using the sqlite3_bind functions.

• Always prefer snprintf instead of sprintf and check its output. It will save you from lots of trouble.

• Make sure you run your FUSE filesystem process in the foreground - it makes debugging easier.

Have you tried breakpoints in gdb? Or a bunch of perror() calls in your code to locate where the errno value that you mentioned comes from?

BTW there is no strtol() call in the code snippet you provided and there are a few macros without their definitions. Also IIRC 13 is the error code for EACCESS.

EDIT:

Something that you may have missed from the FUSE API:

A major exception is that instead of returning an error in 'errno', the operation should return the negated error value (-errno) directly.

You seem to be returning errno as-is.

It's not really an answer, but I got around the issue by running as root. I suspect it has something to do with FUSE, because I had a similar issue when trying to get sshfs to run as a normal user a few months ago.