From d18d86dfe9d1820fc281759e77153b669c6899cd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Maciej=20=C5=BBenczykowski?= <maze@google.com>
Date: Mon, 11 Mar 2024 12:40:06 +0000
Subject: [PATCH] bpfloader: add back support for 4.9-T kernels
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This effectively reverts aosp/2268766

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I42da5c56fe4b69635b9678ff57ade7788c17f7c8
---
 loader/Loader.cpp | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/loader/Loader.cpp b/loader/Loader.cpp
index 6ec7458..5f0fb89 100644
--- a/loader/Loader.cpp
+++ b/loader/Loader.cpp
@@ -483,6 +483,14 @@ static int getSymNameByIdx(ifstream& elfFile, int index, string& name) {
 
 static bool mapMatchesExpectations(const unique_fd& fd, const string& mapName,
                                    const struct bpf_map_def& mapDef, const enum bpf_map_type type) {
+    // bpfGetFd... family of functions require at minimum a 4.14 kernel,
+    // so on 4.9-T kernels just pretend the map matches our expectations.
+    // Additionally we'll get almost equivalent test coverage on newer devices/kernels.
+    // This is because the primary failure mode we're trying to detect here
+    // is either a source code misconfiguration (which is likely kernel independent)
+    // or a newly introduced kernel feature/bug (which is unlikely to get backported to 4.9).
+    if (!isAtLeastKernelVersion(4, 14, 0)) return true;
+
     // Assuming fd is a valid Bpf Map file descriptor then
     // all the following should always succeed on a 4.14+ kernel.
     // If they somehow do fail, they'll return -1 (and set errno),
@@ -572,6 +580,16 @@ static int createMaps(const char* elfPath, ifstream& elfFile, vector<unique_fd>&
         }
 
         enum bpf_map_type type = md[i].type;
+        if (type == BPF_MAP_TYPE_DEVMAP && !isAtLeastKernelVersion(4, 14, 0)) {
+            // On Linux Kernels older than 4.14 this map type doesn't exist, but it can kind
+            // of be approximated: ARRAY has the same userspace api, though it is not usable
+            // by the same ebpf programs.  However, that's okay because the bpf_redirect_map()
+            // helper doesn't exist on 4.9-T anyway (so the bpf program would fail to load,
+            // and thus needs to be tagged as 4.14+ either way), so there's nothing useful you
+            // could do with a DEVMAP anyway (that isn't already provided by an ARRAY)...
+            // Hence using an ARRAY instead of a DEVMAP simply makes life easier for userspace.
+            type = BPF_MAP_TYPE_ARRAY;
+        }
         if (type == BPF_MAP_TYPE_DEVMAP_HASH && !isAtLeastKernelVersion(5, 4, 0)) {
             // On Linux Kernels older than 5.4 this map type doesn't exist, but it can kind
             // of be approximated: HASH has the same userspace visible api.