From ef378c3b8233855497a414b9d67bf22592c928a4 Mon Sep 17 00:00:00 2001 From: Steven Rostedt Date: Tue, 18 Feb 2025 14:59:22 -0500 Subject: scripts/sorttable: Zero out weak functions in mcount_loc table When a function is annotated as "weak" and is overridden, the code is not removed. If it is traced, the fentry/mcount location in the weak function will be referenced by the "__mcount_loc" section. This will then be added to the available_filter_functions list. Since only the address of the functions are listed, to find the name to show, a search of kallsyms is used. Since kallsyms will return the function by simply finding the function that the address is after but before the next function, an address of a weak function will show up as the function before it. This is because kallsyms does not save names of weak functions. This has caused issues in the past, as now the traced weak function will be listed in available_filter_functions with the name of the function before it. At best, this will cause the previous function's name to be listed twice. At worse, if the previous function was marked notrace, it will now show up as a function that can be traced. Note that it only shows up that it can be traced but will not be if enabled, which causes confusion. https://lore.kernel.org/all/20220412094923.0abe90955e5db486b7bca279@kernel.org/ The commit b39181f7c6907 ("ftrace: Add FTRACE_MCOUNT_MAX_OFFSET to avoid adding weak function") was a workaround to this by checking the function address before printing its name. If the address was too far from the function given by the name then instead of printing the name it would print: __ftrace_invalid_address___ The real issue is that these invalid addresses are listed in the ftrace table look up which available_filter_functions is derived from. A place holder must be listed in that file because set_ftrace_filter may take a series of indexes into that file instead of names to be able to do O(1) lookups to enable filtering (many tools use this method). Even if kallsyms saved the size of the function, it does not remove the need of having these place holders. The real solution is to not add a weak function into the ftrace table in the first place. To solve this, the sorttable.c code that sorts the mcount regions during the build is modified to take a "nm -S vmlinux" input, sort it, and any function listed in the mcount_loc section that is not within a boundary of the function list given by nm is considered a weak function and is zeroed out. Note, this does not mean they will remain zero when booting as KASLR will still shift those addresses. To handle this, the entries in the mcount_loc section will be ignored if they are zero or match the kaslr_offset() value. Before: ~# grep __ftrace_invalid_address___ /sys/kernel/tracing/available_filter_functions | wc -l 551 After: ~# grep __ftrace_invalid_address___ /sys/kernel/tracing/available_filter_functions | wc -l 0 Cc: bpf Cc: Masami Hiramatsu Cc: Mark Rutland Cc: Mathieu Desnoyers Cc: Andrew Morton Cc: Peter Zijlstra Cc: Linus Torvalds Cc: Masahiro Yamada Cc: Nathan Chancellor Cc: Nicolas Schier Cc: Zheng Yejian Cc: Martin Kelly Cc: Christophe Leroy Cc: Josh Poimboeuf Cc: Heiko Carstens Cc: Catalin Marinas Cc: Will Deacon Cc: Vasily Gorbik Cc: Alexander Gordeev Link: https://lore.kernel.org/20250218200022.883095980@goodmis.org Signed-off-by: Steven Rostedt (Google) --- scripts/link-vmlinux.sh | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) (limited to 'scripts/link-vmlinux.sh') diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh index 56a077d204cf..59b07fe6fd00 100755 --- a/scripts/link-vmlinux.sh +++ b/scripts/link-vmlinux.sh @@ -177,12 +177,14 @@ mksysmap() sorttable() { - ${objtree}/scripts/sorttable ${1} + ${NM} -S ${1} > .tmp_vmlinux.nm-sort + ${objtree}/scripts/sorttable -s .tmp_vmlinux.nm-sort ${1} } cleanup() { rm -f .btf.* + rm -f .tmp_vmlinux.nm-sort rm -f System.map rm -f vmlinux rm -f vmlinux.map -- cgit v1.2.3