diff options
Diffstat (limited to 'util/nvmutil/nvmutil.c')
| -rw-r--r-- | util/nvmutil/nvmutil.c | 78 |
1 files changed, 60 insertions, 18 deletions
diff --git a/util/nvmutil/nvmutil.c b/util/nvmutil/nvmutil.c index 8a049c41..493a88e3 100644 --- a/util/nvmutil/nvmutil.c +++ b/util/nvmutil/nvmutil.c @@ -1740,21 +1740,35 @@ err_rw_gbe_file_exact: } /* - * Read or write the exact contents of a file, - * along with a buffer, (if applicable) offset, - * and number of bytes to be read. It unifies - * the functionality of read(), pread(), write() - * and pwrite(), with retry-on-EINTR and also - * prevents infinite loop on zero-reads. + * Safe I/O functions wrapping around + * read(), write() and providing a portable + * analog of both pread() and pwrite(). + * These functions are designed for maximum + * robustness, checking NULL inputs, overflowed + * outputs, and all kinds of errors that the + * standard libc functions don't. * - * The pread() and pwrite() functionality are - * provided by yet another portable function, - * prw() - see notes below. + * Looping on EINTR and EAGAIN is supported. + * EINTR/EAGAIN looping is done indefinitely. + */ + +/* + * rw_file_exact() - Read perfectly or die + * + * Read/write, and absolutely insist on an + * absolute read; e.g. if 100 bytes are + * requested, this MUST return 100. + * + * This function will never return zero. + * It will only return below (error), + * or above (success). On error, -1 is + * returned and errno is set accordingly. * - * This must only be used on files. It cannot - * be used on sockets or pipes, because 0-byte - * reads are treated like fatal errors. This - * means that EOF is also considered fatal. + * Zero-byte returns are not allowed. + * It calls rw_file_once(), which will + * re-try on zero-read a finite number + * of times, to prevent infinite loops + * while also having fault tolerance. */ static ssize_t rw_file_exact(int fd, u8 *mem, size_t nrw, @@ -1785,17 +1799,19 @@ rw_file_exact(int fd, u8 *mem, size_t nrw, } /* - * Helper function for rw_file_exact, that - * also does extra error handling pertaining - * to GbE file offsets. + * rw_file_once() - Read less than perfectly + * (and possibly die) * - * May not return all requested bytes (nrw). - * Use rw_file_exact for guaranteed length. + * Read/write, but don't insist on an + * absolute read; e.g. if 100 bytes are + * requested, this may return 80 <-- fine * * This function will never return zero. * It will only return below (error), * or above (success). On error, -1 is * returned and errno is set accordingly. + * + * Zero-byte returns are not allowed. */ static ssize_t rw_file_once(int fd, u8 *mem, size_t nrw, @@ -1804,6 +1820,18 @@ rw_file_once(int fd, u8 *mem, size_t nrw, { ssize_t rv; size_t retries_on_zero = 0; + + /* + * Retries on zero-return. + * + * 10 retries is generous, + * but also conservative. + * This is enough for e.g. + * slow USB flash drives, + * busy NFS servers, etc. + * Any more is too much + * and not of much benefit. + */ size_t max_retries = 10; if (mem == NULL) @@ -1840,6 +1868,7 @@ err_rw_file_once: * * This limitation is acceptable, since nvmutil is * single-threaded. Portability is the main goal. + * If you need real pwrite/pread, just edit prw() * * A fallback is provided for regular read/write. * rw_type can be IO_READ, IO_WRITE, IO_PREAD @@ -1945,6 +1974,8 @@ err_prw: } /* + * Check overflows caused by buggy libc. + * * POSIX can say whatever it wants. * specification != implementation */ @@ -1986,6 +2017,11 @@ err_rw_over_nrw: return -1; } +/* + * lseek_loop() does lseek() but optionally + * on an EINTR/EAGAIN wait loop. Used by prw() + * for setting offsets for positional I/O. + */ static off_t lseek_loop(int fd, off_t off, int whence, int loop_eagain, int loop_eintr) @@ -2001,6 +2037,12 @@ lseek_loop(int fd, off_t off, int whence, return old; } +/* + * If a given error loop is enabled, + * e.g. EINTR or EAGAIN, an I/O operation + * will loop until errno isn't -1 and one + * of these, e.g. -1 and EINTR + */ static int try_err(int loop_err, int errval) { |