[PATCH v5 07/11] net/9p: limit 'msize' to KMALLOC_MAX_SIZE for all transports

From: Christian Schoenebeck
Date: Tue Jul 12 2022 - 12:26:22 EST

This 9p client implementation is yet using linear message buffers for
most message types, i.e. they use kmalloc() et al. for allocating
continuous physical memory pages, which is usually limited to 4MB
buffers. Use KMALLOC_MAX_SIZE though instead of a hard coded 4MB for
constraining this more safely.

Unfortunately we cannot simply replace the existing kmalloc() calls by
vmalloc() ones, because that would yield in non-logical kernel addresses
(for any vmalloc(>4MB) that is) which are in general not accessible by
hosts like QEMU.

In future we would replace those linear buffers by scatter/gather lists
to eventually get rid of this limit (struct p9_fcall's sdata member by
p9_fcall_init() and struct p9_fid's rdir member by

Signed-off-by: Christian Schoenebeck <linux_oss@xxxxxxxxxxxxx>

Hmm, that's a bit too simple, as we also need a bit of headroom for
transport specific overhead. So maybe this has to be handled by each
transport appropriately instead?

net/9p/client.c | 11 +++++++++++
1 file changed, 11 insertions(+)

diff --git a/net/9p/client.c b/net/9p/client.c
index 20054addd81b..fab939541c81 100644
--- a/net/9p/client.c
+++ b/net/9p/client.c
@@ -1042,6 +1042,17 @@ struct p9_client *p9_client_create(const char *dev_name, char *options)
p9_debug(P9_DEBUG_MUX, "clnt %p trans %p msize %d protocol %d\n",
clnt, clnt->trans_mod, clnt->msize, clnt->proto_version);

+ /*
+ * due to linear message buffers being used by client ATM
+ */
+ if (clnt->msize > KMALLOC_MAX_SIZE) {
+ clnt->msize = KMALLOC_MAX_SIZE;
+ pr_info("Limiting 'msize' to %zu as this is the maximum "
+ "supported by this client version.\n",
+ );
+ }
err = clnt->trans_mod->create(clnt, dev_name, options);
if (err)
goto put_trans;