The ftrace selftest code has a trace_direct_tramp() function which it uses as a direct call trampoline. This happens to work on x86, since the direct call's return address is in the usual place, and can be returned to via a RET, but in general the calling convention for direct calls is different from regular function calls, and requires a trampoline written in assembly. On s390, regular function calls place the return address in %r14, and an ftrace patch-site in an instrumented function places the trampoline's return address (which is within the instrumented function) in %r0, preserving the original %r14 value in-place. As a regular C function will return to the address in %r14, using a C function as the trampoline results in the trampoline returning to the caller of the instrumented function, skipping the body of the instrumented function. Note that the s390 issue is not detcted by the ftrace selftest code, as the instrumented function is trivial, and returning back into the caller happens to be equivalent. On arm64, regular function calls place the return address in x30, and an ftrace patch-site in an instrumented function saves this into r9 and places the trampoline's return address (within the instrumented function) in x30. A regular C function will return to the address in x30, but will not restore x9 into x30. Consequently, using a C function as the trampoline results in returning to the trampoline's return address having corrupted x30, such that when the instrumented function returns, it will return back into itself. To avoid future issues in this area, remove the trace_direct_tramp() function, and require that each architecture with direct calls provides a stub trampoline, named ftrace_stub_direct_tramp. This can be written to handle the architecture's trampoline calling convention, and in future could be used elsewhere (e.g. in the ftrace ops sample, to measure the overhead of direct calls), so we may as well always build it in. Link: https://lkml.kernel.org/r/20230321140424.345218-8-revest@chromium.org
Unified · Split
| @@ -32,6 +32,11 @@ ENTRY(ftrace_stub) · 31 unmodified lines | ||
| 32 | 32 | BR_EX %r14 |
| 33 | 33 | ENDPROC(ftrace_stub) |
| 34 | 34 | |
| 35 | +SYM_CODE_START(ftrace_stub_direct_tramp) | |
| 36 | + lgr %r1, %r0 | |
| 37 | + BR_EX %r1 | |
| 38 | +SYM_CODE_END(ftrace_stub_direct_tramp) | |
| 39 | + | |
| 35 | 40 | .macro ftrace_regs_entry, allregs=0 |
| 36 | 41 | stg %r14,(__SF_GPRS+8*8)(%r15) # save traced function caller |
| 37 | 42 | |
| @@ -163,6 +163,11 @@ SYM_INNER_LABEL(ftrace_regs_call, SYM_L_GLOBAL) · 162 unmodified lines | ||
| 163 | 163 | jmp .Lftrace_ret |
| 164 | 164 | SYM_CODE_END(ftrace_regs_caller) |
| 165 | 165 | |
| 166 | +SYM_FUNC_START(ftrace_stub_direct_tramp) | |
| 167 | + CALL_DEPTH_ACCOUNT | |
| 168 | + RET | |
| 169 | +SYM_FUNC_END(ftrace_stub_direct_tramp) | |
| 170 | + | |
| 166 | 171 | #ifdef CONFIG_FUNCTION_GRAPH_TRACER |
| 167 | 172 | SYM_CODE_START(ftrace_graph_caller) |
| 168 | 173 | pushl %eax |
| @@ -309,6 +309,10 @@ SYM_INNER_LABEL(ftrace_regs_caller_end, SYM_L_GLOBAL) · 308 unmodified lines | ||
| 309 | 309 | SYM_FUNC_END(ftrace_regs_caller) |
| 310 | 310 | STACK_FRAME_NON_STANDARD_FP(ftrace_regs_caller) |
| 311 | 311 | |
| 312 | +SYM_FUNC_START(ftrace_stub_direct_tramp) | |
| 313 | + CALL_DEPTH_ACCOUNT | |
| 314 | + RET | |
| 315 | +SYM_FUNC_END(ftrace_stub_direct_tramp) | |
| 312 | 316 | |
| 313 | 317 | #else /* ! CONFIG_DYNAMIC_FTRACE */ |
| 314 | 318 | |
| @@ -413,6 +413,8 @@ int unregister_ftrace_direct(struct ftrace_ops *ops, unsigned long addr, · 412 unmodified lines | ||
| 413 | 413 | int modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr); |
| 414 | 414 | int modify_ftrace_direct_nolock(struct ftrace_ops *ops, unsigned long addr); |
| 415 | 415 | |
| 416 | +void ftrace_stub_direct_tramp(void); | |
| 417 | + | |
| 416 | 418 | #else |
| 417 | 419 | struct ftrace_ops; |
| 418 | 420 | # define ftrace_direct_func_count 0 |
| @@ -786,14 +786,6 @@ static struct fgraph_ops fgraph_ops __initdata = { · 785 unmodified lines | ||
| 786 | 786 | |
| 787 | 787 | #ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS |
| 788 | 788 | static struct ftrace_ops direct; |
| 789 | −#ifndef CALL_DEPTH_ACCOUNT | |
| 790 | −#define CALL_DEPTH_ACCOUNT "" | |
| 791 | −#endif | |
| 792 | − | |
| 793 | −noinline __noclone static void trace_direct_tramp(void) | |
| 794 | −{ | |
| 795 | − asm(CALL_DEPTH_ACCOUNT); | |
| 796 | −} | |
| 797 | 789 | #endif |
| 798 | 790 | |
| 799 | 791 | /* |
| @@ -873,7 +865,7 @@ trace_selftest_startup_function_graph(struct tracer *trace, · 73 unmodified lines | ||
| 873 | 865 | */ |
| 874 | 866 | ftrace_set_filter_ip(&direct, (unsigned long)DYN_FTRACE_TEST_NAME, 0, 0); |
| 875 | 867 | ret = register_ftrace_direct(&direct, |
| 876 | − (unsigned long)trace_direct_tramp); | |
| 868 | + (unsigned long)ftrace_stub_direct_tramp); | |
| 877 | 869 | if (ret) |
| 878 | 870 | goto out; |
| 879 | 871 | |
| @@ -894,7 +886,7 @@ trace_selftest_startup_function_graph(struct tracer *trace, · 14 unmodified lines | ||
| 894 | 886 | unregister_ftrace_graph(&fgraph_ops); |
| 895 | 887 | |
| 896 | 888 | ret = unregister_ftrace_direct(&direct, |
| 897 | − (unsigned long) trace_direct_tramp, | |
| 889 | + (unsigned long)ftrace_stub_direct_tramp, | |
| 898 | 890 | true); |
| 899 | 891 | if (ret) |
| 900 | 892 | goto out; |