bmahler commented on code in PR #519:
URL: https://github.com/apache/mesos/pull/519#discussion_r1528991265
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src/linux/cgroups2.cpp:
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@@ -295,4 +296,152 @@ Try<set<string>> enabled(const string& cgroup)
} // namespace controllers {
+namespace devices {
+
+const int ALLOW_ACCESS = 1;
+
+const int DENY_ACCESS = 0;
+
+// Utility class to construct an eBPF program to whitelist or blacklist
+// select device accesses.
+class DeviceProgram
+{
+public:
+ DeviceProgram() : program{ebpf::Program(BPF_PROG_TYPE_CGROUP_DEVICE)}
+ {
+ program.append({
+ // r2: Type ('c', 'b', '?')
+ BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1, 0),
+ BPF_ALU32_IMM(BPF_AND, BPF_REG_2, 0xFFFF),
+ // r3: Access ('r', 'w', 'm')
+ BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_1, 0),
+ BPF_ALU32_IMM(BPF_RSH, BPF_REG_3, 16),
+ // r4: Major Version
+ BPF_LDX_MEM(BPF_W, BPF_REG_4, BPF_REG_1, 4),
+ // r5: Minor Version
+ BPF_LDX_MEM(BPF_W, BPF_REG_5, BPF_REG_1, 8)});
+ }
+
+ Try<Nothing> allow(const Entry entry) { return addDevice(entry, true); }
+
+ Try<Nothing> deny(const Entry entry) { return addDevice(entry, false); }
+
+ Try<Nothing> addDevice(const Entry entry, bool allow)
+ {
+ // We create a block of bytecode with the format:
+ // 1. [ Type Check ] ⤵
+ // 2. [ Access Check ] ⤵
+ // 3. [ Minor Version Check ] ⤵
+ // 4. [ Major Version Check ] ⤵
+ // 5. [ Allow/Deny Access ] ↓
+ // ↓
+ // 6. [ NEXT BLOCK ] ←←
+ //
+ // Either:
+ // 1. The device access is matched by (1,2,3,4) and the Allow/Deny access
+ // block (5) is executed.
+ // 2. One of (1,2,3,4) does not match the requested access and we skip
+ // to the next block (6).
+
+ // We only check attributes that are not 100% permissive:
+ // - Type != all
+ // - Access != rnw
+ // - Minor != None (AKA "any")
+ // - Major != None (AKA "any")
+ Entry::Selector selector = entry.selector;
+ Entry::Access access = entry.access;
+
+ bool check_type = selector.type != Entry::Selector::Type::ALL;
+ bool check_access = !access.mknod || !access.read || !access.write;
+ bool check_major = selector.major.isSome();
+ bool check_minor = selector.minor.isSome();
+
+ // Number of instructions to the [NEXT BLOCK]. This is used if a check
+ // fails (meaning this whitelist entry does not apply) and we want to
+ // skip the subsequent checks.
+ short jmp_size = 1 + (check_access ? 3 : 0) + (check_type ? 1 : 0) +
+ (check_major ? 1 : 0) + (check_minor ? 1 : 0);
+
+ // Check type (r2) against entry.
+ if (check_type) {
+ int bpf_type = selector.type == Entry::Selector::Type::BLOCK
+ ? BPF_DEVCG_DEV_BLOCK
+ : BPF_DEVCG_DEV_CHAR;
+
+ program.append({ BPF_JMP_IMM(BPF_JNE, BPF_REG_2, bpf_type, jmp_size) });
+ --jmp_size;
+ }
+ // Check access (r3) against entry.
+ if (check_access) {
+ int bpf_access = 0;
+ bpf_access |= access.read ? BPF_DEVCG_ACC_READ : 0;
+ bpf_access |= access.write ? BPF_DEVCG_ACC_WRITE : 0;
+ bpf_access |= access.mknod ? BPF_DEVCG_ACC_MKNOD : 0;
+
+ program.append({
+ BPF_MOV32_REG (BPF_REG_1, BPF_REG_3),
+ BPF_ALU32_IMM (BPF_AND, BPF_REG_1, bpf_access),
+ BPF_JMP_REG (BPF_JNE, BPF_REG_1, BPF_REG_3, (short) (jmp_size -
2))});
+ jmp_size -= 3;
+ }
+ // Check major version (r4) against entry.
+ if (check_major) {
+ program.append({
+ BPF_JMP_IMM(BPF_JNE, BPF_REG_4, (int) selector.major.get(),
jmp_size)});
Review Comment:
would be good to explain these instructions in general, e.g.
```
// Jump to XXX if major (r4) != selector.major.
```
Are we jumping *past* the first instruction of the allow / deny access block
below? If so, it seems like it makes sense to separate out the append of the
BPF_EXIT_INSN instruction and make it more clear in the comments here that
we're jumping directly to that?
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