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bpf - add a bunch of static asserts on size/alignment of struct field types

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It really turns out that till now we've just been lucky to not have
ever used a 64-bit type.

See also discussion on:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69560
(and other places on the internet)

Bug: 190519702
Test: atest, TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ibe74a6f97bdbba490a7848060d07dc3efcee0e68
This commit is contained in:
Maciej Żenczykowski 2021-07-05 13:51:15 -07:00
parent 0bf9219e07
commit df91d2b5b2
1 changed files with 76 additions and 11 deletions
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87
progs/include/bpf_map_def.h
View file

@ -42,16 +42,74 @@
* *
******************************************************************************/
// We currently default to v0.1 format
#ifndef BPFLOADER_VERSION
#define BPFLOADER_VERSION 1u
#endif
// These are the values used if these fields are missing
#define DEFAULT_BPFLOADER_MIN_VER 0u // v0.0 (this is inclusive ie. >= v0.0)
#define DEFAULT_BPFLOADER_MAX_VER 0x10000u // v1.0 (this is exclusive ie. < v1.0)
#define DEFAULT_SIZEOF_BPF_MAP_DEF 32 // v0.0 struct: enum + alignment padding + 7 uint
#define DEFAULT_SIZEOF_BPF_PROG_DEF 20 // v0.0 struct: 4 uint + bool + alignment padding
#define DEFAULT_SIZEOF_BPF_MAP_DEF 32 // v0.0 struct: enum (uint sized) + 7 uint
#define DEFAULT_SIZEOF_BPF_PROG_DEF 20 // v0.0 struct: 4 uint + bool + 3 byte alignment pad
/*
* The bpf_{map,prog}_def structures are compiled for different architectures.
* Once by the BPF compiler for the BPF architecture, and once by a C++
* compiler for the native Android architecture for the bpfloader.
*
* For things to work, their layout must be the same between the two.
* The BPF architecture is platform independent ('64-bit LSB bpf').
* So this effectively means these structures must be the same layout
* on 5 architectures, all of them little endian:
* 64-bit BPF, x86_64, arm and 32-bit x86 and arm
*
* As such for any types we use inside of these structs we must make sure that
* the size and alignment are the same, so the same amount of padding is used.
*
* Currently we only use: bool, enum bpf_map_type and unsigned int.
* Additionally we use char for padding.
*
* !!! WARNING: HERE BE DRAGONS !!!
*
* Be particularly careful with 64-bit integers.
* You will need to manually override their alignment to 8 bytes.
*
* To quote some parts of https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69560
*
* Some types have weaker alignment requirements when they are structure members.
*
* unsigned long long on x86 is such a type.
*
* C distinguishes C11 _Alignof (the minimum alignment the type is guaranteed
* to have in all contexts, so 4, see min_align_of_type) from GNU C __alignof
* (the normal alignment of the type, so 8).
*
* alignof / _Alignof == minimum alignment required by target ABI
* __alignof / __alignof__ == preferred alignment
*
* When in a struct, apparently the minimum alignment is used.
*/
_Static_assert(sizeof(bool) == 1, "sizeof bool != 1");
_Static_assert(__alignof__(bool) == 1, "__alignof__ bool != 1");
_Static_assert(_Alignof(bool) == 1, "_Alignof bool != 1");
_Static_assert(sizeof(char) == 1, "sizeof char != 1");
_Static_assert(__alignof__(char) == 1, "__alignof__ char != 1");
_Static_assert(_Alignof(char) == 1, "_Alignof char != 1");
// This basically verifies that an enum is 'just' a 32-bit int
_Static_assert(sizeof(enum bpf_map_type) == 4, "sizeof enum bpf_map_type != 4");
_Static_assert(__alignof__(enum bpf_map_type) == 4, "__alignof__ enum bpf_map_type != 4");
_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(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");
// We don't currently use any 64-bit types in these structs, so this is purely to document issue.
// Here sizeof & __alignof__ are consistent, but _Alignof is not: compile for 'aosp_cf_x86_phone'
_Static_assert(sizeof(unsigned long long) == 8, "sizeof unsigned long long != 8");
_Static_assert(__alignof__(unsigned long long) == 8, "__alignof__ unsigned long long != 8");
// BPF wants 8, but 32-bit x86 wants 4
//_Static_assert(_Alignof(unsigned long long) == 8, "_Alignof unsigned long long != 8");
/*
* Map structure to be used by Android eBPF C programs. The Android eBPF loader
@ -80,13 +138,16 @@ struct bpf_map_def {
unsigned int gid; // gid_t
unsigned int mode; // mode_t
#if BPFLOADER_VERSION >= 1u
// The following fields were added in version 0.1
unsigned int bpfloader_min_ver; // if missing, defaults to 0, ie. v0.0
unsigned int bpfloader_max_ver; // if missing, defaults to 0x10000, ie. v1.0
#endif
};
// This needs to be updated whenever the above structure definition is expanded.
_Static_assert(sizeof(struct bpf_map_def) == 40, "sizeof struct bpf_map_def != 40");
_Static_assert(__alignof__(struct bpf_map_def) == 4, "__alignof__ struct bpf_map_def != 4");
_Static_assert(_Alignof(struct bpf_map_def) == 4, "_Alignof struct bpf_map_def != 4");
struct bpf_prog_def {
unsigned int uid;
unsigned int gid;
@ -96,10 +157,14 @@ struct bpf_prog_def {
unsigned int max_kver;
bool optional; // program section (ie. function) may fail to load, continue onto next func.
char pad0[3];
#if BPFLOADER_VERSION >= 1u
// The following fields were added in version 0.1
unsigned int bpfloader_min_ver; // if missing, defaults to 0, ie. v0.0
unsigned int bpfloader_max_ver; // if missing, defaults to 0x10000, ie. v1.0
#endif
};
// This needs to be updated whenever the above structure definition is expanded.
_Static_assert(sizeof(struct bpf_prog_def) == 28, "sizeof struct bpf_prog_def != 28");
_Static_assert(__alignof__(struct bpf_prog_def) == 4, "__alignof__ struct bpf_prog_def != 4");
_Static_assert(_Alignof(struct bpf_prog_def) == 4, "_Alignof struct bpf_prog_def != 4");