proc_file_read bug?

From: Thomas Hood (jdthood@mail.com)
Date: Fri Jan 25 2002 - 08:36:32 EST


I don't understand this part of proc_file_read() in
fs/proc/generic.c:

                /* This is a hack to allow mangling of file pos independent
                 * of actual bytes read. Simply place the data at page,
                 * return the bytes, and set `start' to the desired offset
                 * as an unsigned int. - Paul.Russell@rustcorp.com.au
                 */
                n -= copy_to_user(buf, start < page ? page : start, n);
                if (n == 0) {
                        if (retval == 0)
                                retval = -EFAULT;
                        break;
                }

                *ppos += start < page ? (long)start : n; /* Move down the file */
                nbytes -= n;
                buf += n;
                retval += n;

When start >= page, we copy n bytes beginning at start and
increase *ppos by n. Makes sense. But what happens when
start < page? We will copy n bytes starting at page, then
increase *ppos by start!! What sense does that make? If
there's cleverness happening here, someone please document it.

For your convenience, I append the whole existing proc_file_read
function below.

--
Thomas

static ssize_t proc_file_read(struct file * file, char * buf, size_t nbytes, loff_t *ppos) { struct inode * inode = file->f_dentry->d_inode; char *page; ssize_t retval=0; int eof=0; ssize_t n, count; char *start; struct proc_dir_entry * dp;

dp = (struct proc_dir_entry *) inode->u.generic_ip; if (!(page = (char*) __get_free_page(GFP_KERNEL))) return -ENOMEM;

while ((nbytes > 0) && !eof) { count = MIN(PROC_BLOCK_SIZE, nbytes);

start = NULL; if (dp->get_info) { /* * Handle backwards compatibility with the old net * routines. */ n = dp->get_info(page, &start, *ppos, count); if (n < count) eof = 1; } else if (dp->read_proc) { n = dp->read_proc(page, &start, *ppos, count, &eof, dp->data); } else break;

if (!start) { /* * For proc files that are less than 4k */ start = page + *ppos; n -= *ppos; if (n <= 0) break; if (n > count) n = count; } if (n == 0) break; /* End of file */ if (n < 0) { if (retval == 0) retval = n; break; } /* This is a hack to allow mangling of file pos independent * of actual bytes read. Simply place the data at page, * return the bytes, and set `start' to the desired offset * as an unsigned int. - Paul.Russell@rustcorp.com.au */ n -= copy_to_user(buf, start < page ? page : start, n); if (n == 0) { if (retval == 0) retval = -EFAULT; break; }

*ppos += start < page ? (long)start : n; /* Move down the file */ nbytes -= n; buf += n; retval += n; } free_page((unsigned long) page); return retval; }

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