Re: [Libguestfs] [nbdkit PATCH 2/3] filter: Add .can_zero/.can_fua overrides

@@ -348,6 +362,8 @@ static struct nbdkit_next_ops next_ops = { .flush = next_flush, .trim = next_trim, .zero = next_zero, + .can_zero = next_can_zero, + .can_fua = next_can_fua,

@@ -582,6 +628,8 @@ static struct backend filter_functions = { .flush = filter_flush, .trim = filter_trim, .zero = filter_zero, + .can_zero = filter_can_zero, + .can_fua = filter_can_fua,

/* Register and load a filter. */ @@ -626,15 +674,23 @@ filter_register (struct backend *next, size_t index, const char *filename, exit (EXIT_FAILURE); } - /* Since the filter might be much older than the current version of - * nbdkit, only copy up to the self-declared _struct_size of the - * filter and zero out the rest. If the filter is much newer then - * we'll only call the "old" fields. + /* Providing a stable filter ABI is much harder than a stable plugin + * ABI. Any newer filter compiled against a larger struct + * nbdkit_next_ops could potentially dereference fields we did not + * provide; and any older filter compiled against a smaller struct + * nbdkit_filter may miss intercepting functions that are vital to + * avoid corrupting the client-visible data. So for now, we insist + * on an exact match in _struct_size, and promise that + * nbdkit_next_ops won't change size without also changing + * _struct_size. */ size = sizeof f->filter; /* our struct */ - memset (&f->filter, 0, size); - if (size > filter->_struct_size) - size = filter->_struct_size; + if (filter->_struct_size != size) { + fprintf (stderr, + "%s: %s: filter compiled against incompatible nbdkit version\n", + program_name, f->filename); + exit (EXIT_FAILURE); + } memcpy (&f->filter, filter, size);

diff --git a/src/internal.h b/src/internal.h index 3cbfde5..2f03279 100644 --- a/src/internal.h +++ b/src/internal.h @@ -191,6 +191,8 @@ struct backend { int (*flush) (struct backend *, struct connection *conn, uint32_t flags); int (*trim) (struct backend *, struct connection *conn, uint32_t count, uint64_t offset, uint32_t flags); int (*zero) (struct backend *, struct connection *conn, uint32_t count, uint64_t offset, uint32_t flags); + int (*can_zero) (struct backend *, struct connection *conn); + int (*can_fua) (struct backend *, struct connection *conn);

/* plugins.c */ diff --git a/src/plugins.c b/src/plugins.c index dba3e24..3468b6d 100644 --- a/src/plugins.c +++ b/src/plugins.c @@ -358,6 +358,26 @@ plugin_can_trim (struct backend *b, struct connection *conn) return p->plugin.trim != NULL; } +static int +plugin_can_zero (struct backend *b, struct connection *conn) +{ + debug ("can_zero"); + + // We always allow .zero to fall back to .write, so plugins don't + // need to override this + return plugin_can_write (b, conn); +} + +static int +plugin_can_fua (struct backend *b, struct connection *conn) +{ + debug ("can_fua"); + + // TODO - wire FUA flag support into plugins. Until then, this copies + // can_flush, since that's how we emulate FUA + return plugin_can_flush (b, conn); +} + static int plugin_pread (struct backend *b, struct connection *conn, void *buf, uint32_t count, uint64_t offset, uint32_t flags) @@ -535,6 +555,8 @@ static struct backend plugin_functions = { .flush = plugin_flush, .trim = plugin_trim, .zero = plugin_zero, + .can_zero = plugin_can_zero, + .can_fua = plugin_can_fua,