Introduction¶

Here we will cover the architecture pieces that the common function tracingcode relies on for proper functioning. Things are broken down into increasingcomplexity so that you can start simple and at least get basic functionality.

Note that this focuses on architecture implementation details only. If youwant more explanation of a feature in terms of common code, review the commonftrace - Function Tracer file.

Ideally, everyone who wishes to retain performance while supporting tracing intheir kernel should make it all the way to dynamic ftrace support.

Prerequisites¶

Ftrace relies on these features being implemented:

• STACKTRACE_SUPPORT - implementsave_stack_trace()

• TRACE_IRQFLAGS_SUPPORT - implement include/asm/irqflags.h

HAVE_FUNCTION_TRACER¶

You will need to implement the mcount and the ftrace_stub functions.

The exact mcount symbol name will depend on your toolchain. Some call it“mcount”, “_mcount”, or even “__mcount”. You can probably figure it out byrunning something like:

$ echo 'main(){}' | gcc -x c -S -o - - -pg | grep mcount        call    mcount

We’ll make the assumption below that the symbol is “mcount” just to keep thingsnice and simple in the examples.

Keep in mind that the ABI that is in effect inside of the mcount function ishighly architecture/toolchain specific. We cannot help you in this regard,sorry. Dig up some old documentation and/or find someone more familiar thanyou to bang ideas off of. Typically, register usage (argument/scratch/etc...)is a major issue at this point, especially in relation to the location of themcount call (before/after function prologue). You might also want to look athow glibc has implemented the mcount function for your architecture. It mightbe (semi-)relevant.

The mcount function should check the function pointer ftrace_trace_functionto see if it is set to ftrace_stub. If it is, there is nothing for you to do,so return immediately. If it isn’t, then call that function in the same waythe mcount function normally calls __mcount_internal -- the first argument isthe “frompc” while the second argument is the “selfpc” (adjusted to remove thesize of the mcount call that is embedded in the function).

For example, if the functionfoo() callsbar(), when thebar() function callsmcount(), the argumentsmcount() will pass to the tracer are:

• “frompc” - the addressbar() will use to return tofoo()

• “selfpc” - the addressbar() (withmcount() size adjustment)

Also keep in mind that this mcount function will be calleda lot, sooptimizing for the default case of no tracer will help the smooth running ofyour system when tracing is disabled. So the start of the mcount function istypically the bare minimum with checking things before returning. That alsomeans the code flow should usually be kept linear (i.e. no branching in the nopcase). This is of course an optimization and not a hard requirement.

Here is some pseudo code that should help (these functions should actually beimplemented in assembly):

void ftrace_stub(void){        return;}void mcount(void){        /* save any bare state needed in order to do initial checking */        extern void (*ftrace_trace_function)(unsigned long, unsigned long);        if (ftrace_trace_function != ftrace_stub)                goto do_trace;        /* restore any bare state */        return;do_trace:        /* save all state needed by the ABI (see paragraph above) */        unsigned long frompc = ...;        unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE;        ftrace_trace_function(frompc, selfpc);        /* restore all state needed by the ABI */}

Don’t forget to export mcount for modules !

extern void mcount(void);EXPORT_SYMBOL(mcount);

HAVE_FUNCTION_GRAPH_TRACER¶

Deep breath ... time to do some real work. Here you will need to update themcount function to check ftrace graph function pointers, as well as implementsome functions to save (hijack) and restore the return address.

The mcount function should check the function pointers ftrace_graph_return(compare to ftrace_stub) and ftrace_graph_entry (compare toftrace_graph_entry_stub). If either of those is not set to the relevant stubfunction, call the arch-specific function ftrace_graph_caller which in turncalls the arch-specific function prepare_ftrace_return. Neither of thesefunction names is strictly required, but you should use them anyway to stayconsistent across the architecture ports -- easier to compare & contrastthings.

The arguments to prepare_ftrace_return are slightly different than what arepassed to ftrace_trace_function. The second argument “selfpc” is the same,but the first argument should be a pointer to the “frompc”. Typically this islocated on the stack. This allows the function to hijack the return addresstemporarily to have it point to the arch-specific function return_to_handler.That function will simply call the common ftrace_return_to_handler function andthat will return the original return address with which you can return to theoriginal call site.

Here is the updated mcount pseudo code:

void mcount(void){...        if (ftrace_trace_function != ftrace_stub)                goto do_trace;+#ifdef CONFIG_FUNCTION_GRAPH_TRACER+       extern void (*ftrace_graph_return)(...);+       extern void (*ftrace_graph_entry)(...);+       if (ftrace_graph_return != ftrace_stub ||+           ftrace_graph_entry != ftrace_graph_entry_stub)+               ftrace_graph_caller();+#endif        /* restore any bare state */...

Here is the pseudo code for the new ftrace_graph_caller assembly function:

#ifdef CONFIG_FUNCTION_GRAPH_TRACERvoid ftrace_graph_caller(void){        /* save all state needed by the ABI */        unsigned long *frompc = &...;        unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE;        /* passing frame pointer up is optional -- see below */        prepare_ftrace_return(frompc, selfpc, frame_pointer);        /* restore all state needed by the ABI */}#endif

For information on how to implementprepare_ftrace_return(), simply look at thex86 version (the frame pointer passing is optional; see the next section formore information). The only architecture-specific piece in it is the setup ofthe fault recovery table (the asm(...) code). The rest should be the sameacross architectures.

Here is the pseudo code for the new return_to_handler assembly function. Notethat the ABI that applies here is different from what applies to the mcountcode. Since you are returning from a function (after the epilogue), you mightbe able to skimp on things saved/restored (usually just registers used to passreturn values).

#ifdef CONFIG_FUNCTION_GRAPH_TRACERvoid return_to_handler(void){        /* save all state needed by the ABI (see paragraph above) */        void (*original_return_point)(void) = ftrace_return_to_handler();        /* restore all state needed by the ABI */        /* this is usually either a return or a jump */        original_return_point();}#endif

HAVE_FUNCTION_GRAPH_FP_TEST¶

An arch may pass in a unique value (frame pointer) to both the entering andexiting of a function. On exit, the value is compared and if it does notmatch, then it will panic the kernel. This is largely a sanity check for badcode generation with gcc. If gcc for your port sanely updates the framepointer under different optimization levels, then ignore this option.

However, adding support for it isn’t terribly difficult. In your assembly codethat callsprepare_ftrace_return(), pass the frame pointer as the 3rd argument.Then in the C version of that function, do what the x86 port does and pass italong toftrace_push_return_trace() instead of a stub value of 0.

Similarly, when you callftrace_return_to_handler(), pass it the frame pointer.

HAVE_SYSCALL_TRACEPOINTS¶

You need very few things to get the syscalls tracing in an arch.

• Support HAVE_ARCH_TRACEHOOK (see arch/Kconfig).

• Have a NR_syscalls variable in <asm/unistd.h> that provides the numberof syscalls supported by the arch.

• Support the TIF_SYSCALL_TRACEPOINT thread flags.

• Put thetrace_sys_enter() andtrace_sys_exit() tracepoints calls from ptracein the ptrace syscalls tracing path.

• If the system call table on this arch is more complicated than a simple arrayof addresses of the system calls, implement an arch_syscall_addr to returnthe address of a given system call.

• If the symbol names of the system calls do not match the function names onthis arch, define ARCH_HAS_SYSCALL_MATCH_SYM_NAME in asm/ftrace.h andimplement arch_syscall_match_sym_name with the appropriate logic to returntrue if the function name corresponds with the symbol name.

• Tag this arch as HAVE_SYSCALL_TRACEPOINTS.

HAVE_DYNAMIC_FTRACE¶

See scripts/recordmcount.pl for more info. Just fill in the arch-specificdetails for how to locate the addresses of mcount call sites via objdump.This option doesn’t make much sense without also implementing dynamic ftrace.

You will first need HAVE_FUNCTION_TRACER, so scroll your reader back up if yougot over eager.

Once those are out of the way, you will need to implement:

• asm/ftrace.h:

  • MCOUNT_ADDR

  • ftrace_call_adjust()

  • structdyn_arch_ftrace{}

• asm code:

  • mcount() (new stub)

  • ftrace_caller()

  • ftrace_call()

  • ftrace_stub()

• C code:

  • ftrace_dyn_arch_init()

  • ftrace_make_nop()

  • ftrace_make_call()

  • ftrace_update_ftrace_func()

First you will need to fill out some arch details in your asm/ftrace.h.

Define MCOUNT_ADDR as the address of your mcount symbol similar to:

#define MCOUNT_ADDR ((unsigned long)mcount)

Since no one else will have a decl for that function, you will need to:

extern void mcount(void);

You will also need the helper functionftrace_call_adjust(). Most peoplewill be able to stub it out like so:

static inline unsigned long ftrace_call_adjust(unsigned long addr){        return addr;}

<details to be filled>

Lastly you will need the custom dyn_arch_ftrace structure. If you needsome extra state when runtime patching arbitrary call sites, this is theplace. For now though, create an empty struct:

struct dyn_arch_ftrace {        /* No extra data needed */};

With the header out of the way, we can fill out the assembly code. While wedid already create amcount() function earlier, dynamic ftrace only wants astub function. This is because themcount() will only be used during bootand then all references to it will be patched out never to return. Instead,the guts of the oldmcount() will be used to create a newftrace_caller()function. Because the two are hard to merge, it will most likely be a loteasier to have two separate definitions split up by #ifdefs. Same goes fortheftrace_stub() as that will now be inlined inftrace_caller().

Before we get confused anymore, let’s check out some pseudo code so you canimplement your own stuff in assembly:

void mcount(void){        return;}void ftrace_caller(void){        /* save all state needed by the ABI (see paragraph above) */        unsigned long frompc = ...;        unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE;ftrace_call:        ftrace_stub(frompc, selfpc);        /* restore all state needed by the ABI */ftrace_stub:        return;}

This might look a little odd at first, but keep in mind that we will be runtimepatching multiple things. First, only functions that we actually want to tracewill be patched to callftrace_caller(). Second, since we only have one traceractive at a time, we will patch theftrace_caller() function itself to call thespecific tracer in question. That is the point of the ftrace_call label.

With that in mind, let’s move on to the C code that will actually be doing theruntime patching. You’ll need a little knowledge of your arch’s opcodes inorder to make it through the next section.

Every arch has an init callback function. If you need to do something early onto initialize some state, this is the time to do that. Otherwise, this simplefunction below should be sufficient for most people:

int __init ftrace_dyn_arch_init(void){        return 0;}

There are two functions that are used to do runtime patching of arbitraryfunctions. The first is used to turn the mcount call site into a nop (whichis what helps us retain runtime performance when not tracing). The second isused to turn the mcount call site into a call to an arbitrary location (buttypically that isftracer_caller()). See the general function definition inlinux/ftrace.h for the functions:

ftrace_make_nop()ftrace_make_call()

The rec->ip value is the address of the mcount call site that was collectedby the scripts/recordmcount.pl during build time.

The last function is used to do runtime patching of the active tracer. Thiswill be modifying the assembly code at the location of the ftrace_call symbolinside of theftrace_caller() function. So you should have sufficient paddingat that location to support the new function calls you’ll be inserting. Somepeople will be using a “call” type instruction while others will be using a“branch” type instruction. Specifically, the function is:

ftrace_update_ftrace_func()

HAVE_DYNAMIC_FTRACE + HAVE_FUNCTION_GRAPH_TRACER¶

The function grapher needs a few tweaks in order to work with dynamic ftrace.Basically, you will need to:

• update:

  • ftrace_caller()

  • ftrace_graph_call()

  • ftrace_graph_caller()

• implement:

  • ftrace_enable_ftrace_graph_caller()

  • ftrace_disable_ftrace_graph_caller()

<details to be filled>

Quick notes:

• add a nop stub after the ftrace_call location named ftrace_graph_call;stub needs to be large enough to support a call toftrace_graph_caller()

• updateftrace_graph_caller() to work with being called by the newftrace_caller() since some semantics may have changed

• ftrace_enable_ftrace_graph_caller() will runtime patch theftrace_graph_call location with a call toftrace_graph_caller()

• ftrace_disable_ftrace_graph_caller() will runtime patch theftrace_graph_call location with nops