[PATCH AUTOSEL 5.2 56/94] kallsyms: Don't let kallsyms_lookup_size_offset() fail on retrieving the first symbol

From: Sasha Levin
Date: Wed Sep 04 2019 - 11:59:08 EST

From: Marc Zyngier <maz@xxxxxxxxxx>

[ Upstream commit 2a1a3fa0f29270583f0e6e3100d609e09697add1 ]

An arm64 kernel configured with


reports the following kprobe failure:

[ 0.032677] kprobes: failed to populate blacklist: -22
[ 0.033376] Please take care of using kprobes.

It appears that kprobe fails to retrieve the symbol at address
0xffff000010081000, despite this symbol being in System.map:

ffff000010081000 T __exception_text_start

This symbol is part of the first group of aliases in the
kallsyms_offsets array (symbol names generated using ugly hacks in

.long 0x1000 // do_undefinstr
.long 0x1000 // efi_header_end
.long 0x1000 // _stext
.long 0x1000 // __exception_text_start
.long 0x12b0 // do_cp15instr

Looking at the implementation of get_symbol_pos(), it returns the
lowest index for aliasing symbols. In this case, it return 0.

But kallsyms_lookup_size_offset() considers 0 as a failure, which
is obviously wrong (there is definitely a valid symbol living there).
In turn, the kprobe blacklisting stops abruptly, hence the original

A CONFIG_KALLSYMS_ALL kernel wouldn't fail as there is always
some random symbols at the beginning of this array, which are never
looked up via kallsyms_lookup_size_offset.

Fix it by considering that get_symbol_pos() is always successful
(which is consistent with the other uses of this function).

Fixes: ffc5089196446 ("[PATCH] Create kallsyms_lookup_size_offset()")
Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
kernel/kallsyms.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index 95a260f9214b9..136ce049c4ad2 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -263,8 +263,10 @@ int kallsyms_lookup_size_offset(unsigned long addr, unsigned long *symbolsize,
char namebuf[KSYM_NAME_LEN];

- if (is_ksym_addr(addr))
- return !!get_symbol_pos(addr, symbolsize, offset);
+ if (is_ksym_addr(addr)) {
+ get_symbol_pos(addr, symbolsize, offset);
+ return 1;
+ }
return !!module_address_lookup(addr, symbolsize, offset, NULL, namebuf) ||
!!__bpf_address_lookup(addr, symbolsize, offset, namebuf);