commit d2459f373ce41ab0e65483ab42da0e354298b0cd
author Maciej Żenczykowski <maze@google.com> Mon Jan 20 14:38:02 2025 -0800
committer Maciej Żenczykowski <maze@google.com> Mon Jan 20 14:49:44 2025 -0800
tree 8787aa62917a43e6f09df6b549545da427a60f7f
parent 4c0ba60c3690877d8a96734e6dcededf513353f9

bpf: support BPF_F_NO_PREALLOC in bpf_map_def.h

Kernel requires maps of type BPF_MAP_TYPE_LPM_TRIE to not be
preallocated, so default enable the flag for these.

But even for non LPM_TRIE maps allow a negative num_entries
value to ask for 'non preallocated' maps.

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I48d5d5813628c87094c5efa7266b0354b135dd7e

bpf/headers/include/bpf_helpers.h

diff --git a/bpf/headers/include/bpf_helpers.h b/bpf/headers/include/bpf_helpers.h
index 073a349..d817cc3 100644
--- a/bpf/headers/include/bpf_helpers.h
+++ b/bpf/headers/include/bpf_helpers.h
@@ -231,6 +231,14 @@
               (ignore_userdebug).ignore_on_userdebug),                                   \
         "bpfloader min version must be >= 0.33 in order to use ignored_on");
 
+#define ABSOLUTE(x) ((x) < 0 ? -(x) : (x))
+
+#define DEFAULT_BPF_MAP_FLAGS(type, num_entries, mapflags)    \
+    ( (mapflags) |                                            \
+      ((num_entries) < 0 ? BPF_F_NO_PREALLOC : 0) |           \
+      (type == BPF_MAP_TYPE_LPM_TRIE ? BPF_F_NO_PREALLOC : 0) \
+    )
+
 #define DEFINE_BPF_MAP_BASE(the_map, TYPE, keysize, valuesize, num_entries, \
                             usr, grp, md, selinux, pindir, share, minkver,  \
                             maxkver, minloader, maxloader, ignore_eng,      \
@@ -239,8 +247,8 @@
         .type = BPF_MAP_TYPE_##TYPE,                                        \
         .key_size = (keysize),                                              \
         .value_size = (valuesize),                                          \
-        .max_entries = (num_entries),                                       \
-        .map_flags = (mapflags),                                            \
+        .max_entries = ABSOLUTE(num_entries),                               \
+        .map_flags = DEFAULT_BPF_MAP_FLAGS(BPF_MAP_TYPE_##TYPE, num_entries, mapflags), \
         .uid = (usr),                                                       \
         .gid = (grp),                                                       \
         .mode = (md),                                                       \

bpf/headers/include/bpf_map_def.h

diff --git a/bpf/headers/include/bpf_map_def.h b/bpf/headers/include/bpf_map_def.h
index 2d6736c..d67da48 100644
--- a/bpf/headers/include/bpf_map_def.h
+++ b/bpf/headers/include/bpf_map_def.h
@@ -94,6 +94,10 @@
 _Static_assert(_Alignof(enum bpf_map_type) == 4, "_Alignof enum bpf_map_type != 4");
 
 // Linux kernel requires sizeof(int) == 4, sizeof(void*) == sizeof(long), sizeof(long long) == 8
+_Static_assert(sizeof(int) == 4, "sizeof int != 4");
+_Static_assert(__alignof__(int) == 4, "__alignof__ int != 4");
+_Static_assert(_Alignof(int) == 4, "_Alignof int != 4");
+
 _Static_assert(sizeof(unsigned int) == 4, "sizeof unsigned int != 4");
 _Static_assert(__alignof__(unsigned int) == 4, "__alignof__ unsigned int != 4");
 _Static_assert(_Alignof(unsigned int) == 4, "_Alignof unsigned int != 4");
@@ -155,7 +159,7 @@
     enum bpf_map_type type;
     unsigned int key_size;
     unsigned int value_size;
-    unsigned int max_entries;
+    int max_entries;  // negative means BPF_F_NO_PREALLOC, but *might* not work with S
     unsigned int map_flags;
 
     // The following are not supported by the Android bpfloader: