Hello, all. In the previous discussion, I'm trying to reserve some space (such as 16-bytes) before function to store the information about the function: https://lore.kernel.org/bpf/CADxym3ZfHv_VdgopE5TBQxhO7RrPTVm83VW07c8bAywp404QPw@xxxxxxxxxxxxxx/T/#u And let me describe it here again. For example, we have a function "do_test", the layout is just like this: -------------> 16-bytes -------------> do_test -------------> __fentry__(nop) Then, we can alloc a memory, store some information that we need, and store the pointer of this memory to "do_test - 8". And then, we can get the function information with "ip - 8" in bpf trampoline or ftrace handler. After I dig into the code, I find that this space is already reserved with the config: CONFIG_MITIGATION_CALL_DEPTH_TRACKING CONFIG_CALL_THUNKS CALL_PADDING And the reserved space before the function is 16 bytes. According to my tests, 9 bytes is used by the call depth tracking if the CPU support it, and the insn is just like this: __pfx_do_test: nop nop nop nop nop nop nop (7 bytes) sarq (9 bytes) do_test: xxx And all the calls to do_test will be redirected to __pfx_do_test + 7. I think that there are still 7 bytes for me to use, which is enough. In fact, 4 bytes is enough for me, as we can allocate a function meta array, and store the index to the reserved space. However, the other 5-bytes will be consumed if CFI_CLANG is enabled, and the space is not enough anymore in this case, and the insn will be like this: __cfi_do_test: mov (5byte) nop nop (2 bytes) sarq (9 bytes) do_test: xxx A method that I think is that we can use such "function meta" feature only if the call depth tracking or the CFI is not enabled. (I'm not sure if the CFI is a commonly used feature.) I would appreciate some advice :/ Thanks! Menglong Dong
