program: don't return error when kmod BTF is disabled

In kernels where the flag CONFIG_DEBUG_INFO_BTF_MODULES is not set,
including kernels 5.10 and below (because the flag was introduced in 5.11),
loading a program that attaches to kernel module functions and relied on
CORE failed because the module's BTF is not available.

This fix allows the program to run, obviously only as long as it only
relies on the kernel's BTF and not on the specific module's BTF.

Fixes: #1436
Signed-off-by: Ori Shussman <[email protected]>

Author: orishuss

btf/kernel.go

@@ -63,6 +63,7 @@ func LoadKernelSpec() (*Spec, error) {
 //
 // Defaults to /sys/kernel/btf/<module>.
 // Returns an error wrapping ErrNotSupported if BTF is not enabled.
+// Returns an error wrapping fs.ErrNotExist if BTF for the specific module doesn't exist.
 func LoadKernelModuleSpec(module string) (*Spec, error) {
 	kernelBTF.RLock()
 	spec := kernelBTF.modules[module]

linker.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"math"
 	"slices"
 
@@ -148,10 +149,13 @@ func applyRelocations(insns asm.Instructions, targets []*btf.Spec, kmodName stri
 
 		if kmodName != "" {
 			kmodTarget, err := btf.LoadKernelModuleSpec(kmodName)
-			if err != nil {
+			// Ignore ErrNotExists to cater to kernels which have CONFIG_DEBUG_INFO_BTF_MODULES disabled.
+			if err != nil && !errors.Is(err, fs.ErrNotExist) {
 				return fmt.Errorf("load kernel module spec: %w", err)
 			}
-			targets = append(targets, kmodTarget)
+			if err == nil {
+				targets = append(targets, kmodTarget)
+			}
 		}
 	}