From: Paul Burton paulburton@kernel.org
commit f8fffebdea752a25757b906f3dffecf1a59a6194 upstream.
Commit 716850ab104d ("MIPS: eBPF: Initial eBPF support for MIPS32 architecture.") enabled our eBPF JIT for MIPS32 kernels, whereas it has previously only been availailable for MIPS64. It was my understanding at the time that the BPF test suite was passing & JITing a comparable number of tests to our cBPF JIT [1], but it turns out that was not the case.
The eBPF JIT has a number of problems on MIPS32:
- Most notably various code paths still result in emission of MIPS64 instructions which will cause reserved instruction exceptions & kernel panics when run on MIPS32 CPUs.
- The eBPF JIT doesn't account for differences between the O32 ABI used by MIPS32 kernels versus the N64 ABI used by MIPS64 kernels. Notably arguments beyond the first 4 are passed on the stack in O32, and this is entirely unhandled when JITing a BPF_CALL instruction. Stack space must be reserved for arguments even if they all fit in registers, and the callee is free to assume that stack space has been reserved for its use - with the eBPF JIT this is not the case, so calling any function can result in clobbering values on the stack & unpredictable behaviour. Function arguments in eBPF are always 64-bit values which is also entirely unhandled - the JIT still uses a single (32-bit) register per argument. As a result all function arguments are always passed incorrectly when JITing a BPF_CALL instruction, leading to kernel crashes or strange behavior.
- The JIT attempts to bail our on use of ALU64 instructions or 64-bit memory access instructions. The code doing this at the start of build_one_insn() incorrectly checks whether BPF_OP() equals BPF_DW, when it should really be checking BPF_SIZE() & only doing so when BPF_CLASS() is one of BPF_{LD,LDX,ST,STX}. This results in false positives that cause more bailouts than intended, and that in turns hides some of the problems described above.
- The kernel's cBPF->eBPF translation makes heavy use of 64-bit eBPF instructions that the MIPS32 eBPF JIT bails out on, leading to most cBPF programs not being JITed at all.
Until these problems are resolved, revert the enabling of the eBPF JIT on MIPS32 done by commit 716850ab104d ("MIPS: eBPF: Initial eBPF support for MIPS32 architecture.").
Note that this does not undo the changes made to the eBPF JIT by that commit, since they are a useful starting point to providing MIPS32 support - they're just not nearly complete.
[1] https://lore.kernel.org/linux-mips/MWHPR2201MB13583388481F01A422CE7D66D4410@...
Signed-off-by: Paul Burton paulburton@kernel.org Fixes: 716850ab104d ("MIPS: eBPF: Initial eBPF support for MIPS32 architecture.") Cc: Daniel Borkmann daniel@iogearbox.net Cc: Hassan Naveed hnaveed@wavecomp.com Cc: Tony Ambardar itugrok@yahoo.com Cc: bpf@vger.kernel.org Cc: netdev@vger.kernel.org Cc: stable@vger.kernel.org # v5.2+ Cc: linux-mips@vger.kernel.org Cc: linux-kernel@vger.kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/mips/Kconfig | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/arch/mips/Kconfig +++ b/arch/mips/Kconfig @@ -46,7 +46,7 @@ config MIPS select HAVE_ARCH_TRACEHOOK select HAVE_ARCH_TRANSPARENT_HUGEPAGE if CPU_SUPPORTS_HUGEPAGES select HAVE_ASM_MODVERSIONS - select HAVE_EBPF_JIT if (!CPU_MICROMIPS) + select HAVE_EBPF_JIT if (64BIT && !CPU_MICROMIPS) select HAVE_CONTEXT_TRACKING select HAVE_COPY_THREAD_TLS select HAVE_C_RECORDMCOUNT