ftrace_init doesn't patch _mcount calls on boot

[Varad Gautam]

Hi,

I'm observing the following on qemu with CONFIG_DYNAMIC_FTRACE=y:

root at debian:/sys/kernel/debug/tracing# echo function > current_tracer
[   73.674265] (____ptrval____): expected (00000013 00000013) but got 
(00006097 a38080e7)
[   73.675144] ------------[ ftrace bug ]------------
[   73.675665] ftrace failed to modify
[   73.676548] [<ffffffe000002010>] _stext+0x10/0x3c
[   73.677641]  actual:   97:60:00:00:e7:80:80:a3
[   73.678534] Setting ftrace call site to call ftrace function
[   73.678842] ftrace record flags: 80000001
[   73.679088]  (1)
[   73.679088]  expected tramp: ffffffe000008020
[   73.679687] ------------[ cut here ]------------

This comes from ftrace_check_current_call(), which ensures that the 
patch target location is set to NOPs before patching it with a 
`auipc/jalr MCOUNT_ADDR` sequence when ftrace is enabled 
(ftrace_nop_initialize -> ftrace_init_nop -> ftrace_make_nop -> 
ftrace_check_current_call).

Dumping a few function prologues from memory before enabling ftrace, I 
see that boot-time patching didn't work as expected. The ftrace_stub 
call sequence still exists, which should have been patched over by NOPs.

(gdb) x/10i blk_put_request
    [...]
    0xffffffe0003169ba <blk_put_request+14>:     auipc   ra,0xffcf1
    0xffffffe0003169be <blk_put_request+18>:     jalr    446(ra)

(gdb) disas blk_put_request
Dump of assembler code for function blk_put_request:
    [...]
    0xffffffe0003169ba <+14>:    auipc   ra,0xffcf1
    0xffffffe0003169be <+18>:    jalr    446(ra) # 0xffffffe000007b78 
<ftrace_stub>

Digging further, I find that the copy_to_kernel_nofault() wrapped in 
patch_insn_write() returns without the write going through. Replacing 
this with a memcpy however, makes ftrace work as expected.

I'll be looking into qemu's emulation for copy_to_kernel_nofault() next, 
but I'd like to check if someone else can reproduce this behavior with 
CONFIG_DYNAMIC_FTRACE=y.

Kernel : 
https://git.kernel.org/pub/scm/linux/kernel/git/riscv/linux.git/commit/?h=for-next&id=7d95a88f9254b711a3a95106fc73f6a3a9866a40

Thanks,
Varad

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