[PATCH 2/2] drbd: increase maximum CPU mask size

The cpu-mask resource option is limited to DRBD_CPU_MASK_SIZE - 1, i.e.
31 characters. bitmap_parse() reads the string as comma separated chunks
of up to eight hex digits, each describing 32 bits, so 31 characters
describe at most 112 CPUs. A full mask cannot be expressed on anything
larger, which by now includes most two-socket machines.

Sparse masks can still reach higher CPU numbers by padding with zero
chunks - "8,0,0,0" selects CPU 99 - but that only helps when few bits
are set.

Userspace has already moved on. drbd-utils carries its own copy of these
headers, where DRBD_CPU_MASK_SIZE has been 256 since 2023 ("drbd:
increase maximum CPU mask size" by Joel Colledge, in LINBIT's
drbd-headers repository). A drbdsetup built against those headers is
rejected with -ERANGE by validate_nla() as soon as the mask does not fit
in the 31 characters this driver accepts.

Raise the limit to match userspace. 255 characters allow a full mask for
908 CPUs: a leading chunk of three hex digits followed by 28 chunks of
eight.

The cpu_mask member of struct res_opts grows accordingly. Two instances
live on the stack, in drbd_adm_resource_opts() and in
drbd_adm_new_resource(), which is uncritical at this size.

This only relaxes an upper bound, so an old drbdsetup keeps working
against a kernel with this change, and a new drbdsetup keeps working
against a kernel without it as long as the mask string still fits in the
old limit.

The new size will likely be too small again eventually. Describing the
mask with bitmap_parselist() would avoid that, but the list and hex
formats are ambiguous for the same input - "8" means CPU 3 in one and
CPU 8 in the other - so it would have to be a separate attribute rather
than a reinterpretation of this one.

Signed-off-by: Ionut Nechita <[email protected]>
---
 include/uapi/linux/drbd.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/drbd.h b/include/uapi/linux/drbd.h
index cf1ec3eb872f..1327ce1006ac 100644
--- a/include/uapi/linux/drbd.h
+++ b/include/uapi/linux/drbd.h
@@ -368,7 +368,12 @@ enum write_ordering_e {
 #define DRBD_MD_INDEX_FLEX_EXT -2
 #define DRBD_MD_INDEX_FLEX_INT -3
 
-#define DRBD_CPU_MASK_SIZE 32
+/*
+ * Maximum size of the cpu-mask string, including the terminating NUL.
+ * The netlink policy accepts DRBD_CPU_MASK_SIZE - 1 characters, which is
+ * enough for a full mask covering 908 CPUs.
+ */
+#define DRBD_CPU_MASK_SIZE 256
 
 /**
  * struct drbd_genlmsghdr - DRBD specific header used in NETLINK_GENERIC requests
-- 
2.55.0