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When a userspace program loads an eBPF program, the kernel checks its instructions with the verifier before allowing execution. The accepted program then runs through an interpreter or, where the architecture and system configuration support it, a just-in-time (JIT) compiler that translates it into native instructions. If the program needs shared state, it can use permitted helper functions to access typed kernel-managed BPF maps; userspace can operate on those maps through the BPF system call interface.
What happens when an eBPF program is loaded?
Think of eBPF execution as a handoff between three distinct pieces: the verifier decides whether the loaded instructions meet the kernel’s safety rules, the interpreter or JIT provides an execution path, and maps hold state that programs and userspace may need to share. Verification is not compilation, and maps are not ordinary local variables.
- Userspace prepares and loads the program. A loader submits the program through the BPF interface. The exact loader workflow is tool-specific; the kernel-side stages discussed here do not depend on one particular loader.
- The verifier checks the instructions. It analyzes control flow and tracked state, including registers and stack contents, and rejects programs when it cannot establish that required safety conditions hold.
- The kernel selects an execution path. Depending on system support and configuration, the program can run through the interpreter or a JIT-compiled native-code path.
- The program accesses state as needed. Permitted helper calls let it interact with maps, while userspace can perform map operations through the BPF system call interface.
The kernel’s BPF documentation is explicitly a work in progress. Details can vary by kernel version, architecture, configuration, and program type; consult the documentation and behavior for the target system rather than assuming every eBPF program follows identical rules. Linux BPF documentation index
What does the verifier check?
The verifier analyzes eBPF instructions rather than proving that the original source code is correct. It follows relevant instruction paths while tracking state such as whether a register is uninitialized, contains a scalar, or holds a typed pointer. It checks matters including memory-access bounds and initialization, valid pointer use, and constraints on function-call arguments. The details of allowed context access and helper calls depend on the program type. Linux verifier documentation
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Why values and branches matter
For scalar values, the verifier tracks signed and unsigned ranges as well as known and unknown bits. A conditional branch can refine what is known on each path. For pointers, it tracks bases and offsets to reason about whether an access is within bounds and properly aligned. A program may therefore fail verification even if its C source looks sensible: the verifier must be able to establish safety from the eBPF instructions and state it tracks.
Why a map lookup needs a NULL check
A map lookup can produce either a pointer to a map value or NULL. A program must establish that the pointer is non-NULL before dereferencing it. Likewise, a read from an uninitialized stack location or an invalid context-field access can cause rejection. These are checks on the loaded program’s possible execution paths, not a general guarantee that arbitrary source code is bug-free.
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Does eBPF always run as native machine code?
No. Linux documents both interpreter execution and JIT compilation. In the interpreter path, execution goes through __bpf_prog_run(). With a supported JIT path, the kernel translates eBPF instructions into native machine instructions for the target architecture. Linux documentation on Classic BPF and eBPF
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The interpreter runs the eBPF program without translating it into native instructions. A JIT compiler performs that translation; it does not replace the verifier or make an otherwise rejected program acceptable. Whether a particular program is JITed depends on the architecture and system configuration, so “eBPF always runs as native code” is not an accurate general description.
What architecture-specific JIT examples show
The kernel documentation illustrates how eBPF registers can map to hardware registers on x86-64 and shows example generated machine code. Such examples explain a possible translation, not a promise that other architectures produce the same instructions, use the same register mapping, or have identical JIT support. Classic BPF vs eBPF documentation
What are BPF maps, and how does a program use them?
BPF maps are kernel-managed storage objects with defined types and semantics. The kernel documentation describes them as “generic storage of different types for sharing data between kernel and user space.” Programs access maps through permitted helper functions; userspace can create maps and look up, update, or delete entries through the bpf system call interface. Linux BPF maps documentation
Choose a map for the access pattern
Map families include arrays, hashes, per-CPU variants, queues, stacks, LPM tries, and specialized maps. Their behavior differs, so the appropriate type depends on the data operation and use case; no single map type is universally best. The kernel’s map documentation describes the available families and their semantics. BPF map types and operations
Map lifetime depends on references
Userspace receives a file descriptor when it creates a map. Map lifetime is tied to references and the particular setup in use, so it should not be described as either automatically permanent or always process-local. The kernel documentation gives an example in which maps held by open file descriptors are deleted automatically when the process exits; setups involving other references, such as pinning, require considering those references as well. BPF maps and their lifetime
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why the program type and target kernel matter
eBPF is not one interchangeable execution environment. Program type governs which context fields a program may access and which helpers or functions it may call. A verifier rule or map operation that applies to one use case should not be assumed to apply to another. For implementation decisions, check the documentation for the relevant program type and the kernel version, architecture, and configuration on the target system. Verifier rules and program-specific constraints
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