
Bugzilla Link	6937
Resolution	DUPLICATE
Resolved on	Apr 25, 2010 13:01
Version	unspecified
OS	Linux
Blocks	llvm/llvm-project#4440
CC	@efriedma-quic

Extended Description

The Linux kernel fails to build now, and there is no CONFIG_ to turn off this "feature": net/ipv4/netfilter/ip_tables.c:72:9: error: fields must have a constant size: 'variable length array in structure' extension
will never be supported
return xt_alloc_initial_table(ipt, IPT);
^
In file included from net/ipv4/netfilter/ip_tables.c:31:
net/ipv4/netfilter/../../netfilter/xt_repldata.h:14:26: note: instantiated from:
struct type##_standard entries[nhooks]; \

Here is the header: /*

Today's hack: quantum tunneling in structs
'entries' and 'term' are never anywhere referenced by word in code. In fact,
they serve as the hanging-off data accessed through repl.data[]. */

define xt_alloc_initial_table(type, typ2) ({ \

unsigned int hook_mask = info->valid_hooks; \
unsigned int nhooks = hweight32(hook_mask); \
unsigned int bytes = 0, hooknum = 0, i = 0; \
struct { \
    struct type##_replace repl; \
    struct type##_standard entries[nhooks]; \
    struct type##_error term; \
} *tbl = kzalloc(sizeof(*tbl), GFP_KERNEL); \
if (tbl == NULL) \
    return NULL; \

I don't think this is much different from VLAs, but instead of the array size depending on function parameter, it depends on a local variable. It also isn't much different from variable-sized structs, it could convert the last 2 fields into [0xi8], and calculate the sizeof() at runtime.

Here is a workaround for the C code, clang could do this automatically: diff --git a/net/netfilter/xt_repldata.h b/net/netfilter/xt_repldata.h index 6efe4e5..e61924d 100644 --- a/net/netfilter/xt_repldata.h +++ b/net/netfilter/xt_repldata.h @@ -11,13 +11,16 @@ unsigned int bytes = 0, hooknum = 0, i = 0; \ struct { \ struct type##_replace repl; \

struct type##_standard entries[nhooks]; \
struct type##_error term; \
} tbl = kzalloc(sizeof(tbl), GFP_KERNEL); \
char rest_of_data[];\
} *tbl = kzalloc(sizeof(struct type##_replace)+\
nhooks*sizeof(struct type##_standard)+\
sizeof(struct type##_error), GFP_KERNEL);\ if (tbl == NULL) \ return NULL; \
struct type##_standard entries = (struct type##_standard)&tbl->rest_of_data;\
struct type##_error term = (struct type##_error)((char)&tbl->rest_of_data + nhookssizeof(struct type##_standard)); strncpy(tbl->repl.name, info->name, sizeof(tbl->repl.name)); \
tbl->term = (struct type##_error)typ2##_ERROR_INIT; \
term = (struct type##_error)typ2##_ERROR_INIT; \ tbl->repl.valid_hooks = hook_mask; \ tbl->repl.num_entries = nhooks + 1; \ tbl->repl.size = nhooks sizeof(struct type##_standard) + \ @@ -27,7 +30,7 @@ continue; \ tbl->repl.hook_entry[hooknum] = bytes; \ tbl->repl.underflow[hooknum] = bytes; \
tbl->entries[i++] = (struct type##_standard) \
entries[i++] = (struct type##_standard) \ typ2##_STANDARD_INIT(NF_ACCEPT); \ bytes += sizeof(struct type##_standard); \ } \

llvm / llvm-project

clang: support "quantum tunneling" #7309

Extended Description

define xt_alloc_initial_table(type, typ2) ({ \