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LoongArch: BPF: Save return address register ra to t0 before trampoline

Modify the build_prologue() function to ensure the return address
register ra is saved to t0 before entering trampoline operations.
This change ensures the accurate return address handling when a BPF
program calls another BPF program, preventing errors in the BPF-to-BPF
call chain.

Cc: stable@vger.kernel.org
Fixes: 677e6123e3d2 ("LoongArch: BPF: Disable trampoline for kernel module function trace")
Signed-off-by: Chenghao Duan <duanchenghao@kylinos.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

authored by

Chenghao Duan and committed by
Huacai Chen d314e1f4 eb71f5c4

+1
+1
arch/loongarch/net/bpf_jit.c
··· 139 139 stack_adjust = round_up(stack_adjust, 16); 140 140 stack_adjust += bpf_stack_adjust; 141 141 142 + move_reg(ctx, LOONGARCH_GPR_T0, LOONGARCH_GPR_RA); 142 143 /* Reserve space for the move_imm + jirl instruction */ 143 144 for (i = 0; i < LOONGARCH_LONG_JUMP_NINSNS; i++) 144 145 emit_insn(ctx, nop);