While working on rewriting function graph tracer, I found a design
flaw in the code. Commit 03274a3ffb449 ("tracing/fgraph: Adjust fgraph depth
before calling trace return callback") tried to fix a bug that caused
interrupts not to be traced if max_depth was set to 1 (for debugging
NO_HZ_FULL), because if it came in just as another function that was being
traced that had a depth of 1 was exiting. This caused the timing of being in
the kernel to be off. The fix was simply to move the calling of the return
function after the updating of curr_ret_stack index as that was what was
being used.
The change log even says that it was safe to do this, but unfortunately it
was not, and the barrier() there was specifically *for* that function (which
shows why one should document barriers).
The problem is that the return callback may still be using what's on the
shadow stack and by changing the shadow stack pointer, it may allow for
another function being traced to overwrite that data. Note, if this happens,
it will only cause garbage data to be traced and will not affect the actual
operations of the kernel (ie. it wont crash).
Unfortunately just reverting that will bring back the old bug. The real way
to fix that old bug is to create another counter to handle the depth, but
this means that we need to change all the architectures that implement
function graph tracing (that's 12 of them). Luckily, I need to do this
anyway in my re-design so this is a good thing.
Since all the archictectures do basicall the same thing to prepare the
function graph trace to be traced, I made a generic function that they all
can use and simplified the logic of all the architectures. Then I'm able to
fix the design problem in one place.
I pushed this code up to let zero-day have a whack at it, and I also
downloaded the latest 8.1.0 cross compilers for all the archs that are
affected and compiled tested them all (and got rid of all the warnings
I introduced as well).
I marked this all for stable, but in reality it may not need to be ported
as it will probably not be trivial to do so, becaues you'll need to also
fix the architectures that are no longer used (although do we care about
them?). But if someone really cares about correct timings of the function
graph profiler when the options/graph-time is set to zero, then be my
guest.
Feel free to test this! I'll be pushing this to linux-next and let it
sit there a week or so before pushing it to Linus.
git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git
ftrace/urgent
Head SHA1: 1ffd62e282a649483afb0bb4d76f7244b3c10075
Steven Rostedt (VMware) (18):
function_graph: Create function_graph_enter() to consolidate architecture code
x86/function_graph: Simplify with function_graph_entry()
ARM: function_graph: Simplify with function_graph_entry()
arm64: function_graph: Simplify with function_graph_entry()
microblaze: function_graph: Simplify with function_graph_entry()
MIPS: function_graph: Simplify with function_graph_entry()
nds32: function_graph: Simplify with function_graph_entry()
parisc: function_graph: Simplify with function_graph_entry()
powerpc/function_graph: Simplify with function_graph_entry()
riscv/function_graph: Simplify with function_graph_entry()
s390/function_graph: Simplify with function_graph_entry()
sh/function_graph: Simplify with function_graph_entry()
sparc/function_graph: Simplify with function_graph_entry()
function_graph: Make ftrace_push_return_trace() static
function_graph: Use new curr_ret_depth to manage depth instead of curr_ret_stack
function_graph: Move return callback before update of curr_ret_stack
function_graph: Reverse the order of pushing the ret_stack and the callback
function_graph: Have profiler use curr_ret_stack and not depth
----
arch/arm/kernel/ftrace.c | 17 +------------
arch/arm64/kernel/ftrace.c | 15 +----------
arch/microblaze/kernel/ftrace.c | 15 ++---------
arch/mips/kernel/ftrace.c | 14 ++---------
arch/nds32/kernel/ftrace.c | 18 ++-----------
arch/parisc/kernel/ftrace.c | 17 +++----------
arch/powerpc/kernel/trace/ftrace.c | 15 ++---------
arch/riscv/kernel/ftrace.c | 14 ++---------
arch/s390/kernel/ftrace.c | 13 ++--------
arch/sh/kernel/ftrace.c | 16 ++----------
arch/sparc/kernel/ftrace.c | 11 +-------
arch/x86/kernel/ftrace.c | 15 +----------
include/linux/ftrace.h | 4 +--
include/linux/sched.h | 1 +
kernel/trace/ftrace.c | 7 ++++--
kernel/trace/trace_functions_graph.c | 49 ++++++++++++++++++++++++++++--------
16 files changed, 67 insertions(+), 174 deletions(-)
