[PATCH 3/7] btrfs: accessors: inline eb bounds check and factor out the error report

David Sterba
2025-07-01T17:24:09Z

There's a check in each set/get helper if the requested range is within
extent buffer bounds, and if it's not then report it. This was in an
ASSERT statement so with CONFIG_BTRFS_ASSERT this crashes right away, on
other configs this is only reported but reading out of the bounds is
done anyway. There are currently no known reports of this particular
condition failing.

There are some drawbacks though. The behaviour dependence on the
assertions being compiled in or not and a less visible effect of
inlining report_setget_bounds() into each helper.

As the bounds check is expected to succeed almost always it's ok to
inline it but make the report a function and move it out of the helper
completely (__cold puts it to a different section). This also skips
reading/writing the requested range in case it fails.

This improves stack usage significantly:

btrfs_get_16 -48 (80 -> 32)
btrfs_get_32 -48 (80 -> 32)
btrfs_get_64 -48 (80 -> 32)
btrfs_get_8 -48 (72 -> 24)
btrfs_set_16 -56 (88 -> 32)
btrfs_set_32 -56 (88 -> 32)
btrfs_set_64 -56 (88 -> 32)
btrfs_set_8 -48 (80 -> 32)

NEW (48):
report_setget_bounds 48
LOST/NEW DELTA: +48
PRE/POST DELTA: -360

Same as .ko size:

text data bss dec hex filename
1456079 115665 16088 1587832 183a78 pre/btrfs.ko
1454951 115665 16088 1586704 183610 post/btrfs.ko

DELTA: -1128

Signed-off-by: David Sterba <dsterba@suse.com>
---
fs/btrfs/accessors.c | 29 +++++++++++++++--------------
1 file changed, 15 insertions(+), 14 deletions(-)

diff --git a/fs/btrfs/accessors.c b/fs/btrfs/accessors.c
index 2e90b9b14e73f4..a7b6b2d7bde224 100644
--- a/fs/btrfs/accessors.c
+++ b/fs/btrfs/accessors.c
@@ -9,20 +9,15 @@
#include "fs.h"
#include "accessors.h"

-static bool check_setget_bounds(const struct extent_buffer *eb,
- const void *ptr, unsigned off, int size)
+static void __cold report_setget_bounds(const struct extent_buffer *eb,
+ const void *ptr, unsigned off, int size)
{
- const unsigned long member_offset = (unsigned long)ptr + off;
+ unsigned long member_offset = (unsigned long)ptr + off;

- if (unlikely(member_offset + size > eb->len)) {
- btrfs_warn(eb->fs_info,
- "bad eb member %s: ptr 0x%lx start %llu member offset %lu size %d",
- (member_offset > eb->len ? "start" : "end"),
- (unsigned long)ptr, eb->start, member_offset, size);
- return false;
- }
-
- return true;
+ btrfs_warn(eb->fs_info,
+ "bad eb member %s: ptr 0x%lx start %llu member offset %lu size %d",
+ (member_offset > eb->len ? "start" : "end"),
+ (unsigned long)ptr, eb->start, member_offset, size);
}

/*
@@ -56,7 +51,10 @@ u##bits btrfs_get_##bits(const struct extent_buffer *eb, \
const int part = eb->folio_size - oil; \
u8 lebytes[sizeof(u##bits)]; \
\
- ASSERT(check_setget_bounds(eb, ptr, off, sizeof(u##bits))); \
+ if (unlikely(member_offset + sizeof(u##bits) > eb->len)) { \
+ report_setget_bounds(eb, ptr, off, sizeof(u##bits)); \
+ return 0; \
+ } \
if (INLINE_EXTENT_BUFFER_PAGES == 1 || likely(sizeof(u##bits) <= part)) \
return get_unaligned_le##bits(kaddr + oil); \
\
@@ -76,7 +74,10 @@ void btrfs_set_##bits(const struct extent_buffer *eb, void *ptr, \
const int part = eb->folio_size - oil; \
u8 lebytes[sizeof(u##bits)]; \
\
- ASSERT(check_setget_bounds(eb, ptr, off, sizeof(u##bits))); \
+ if (unlikely(member_offset + sizeof(u##bits) > eb->len)) { \
+ report_setget_bounds(eb, ptr, off, sizeof(u##bits)); \
+ return; \
+ } \
if (INLINE_EXTENT_BUFFER_PAGES == 1 || \
likely(sizeof(u##bits) <= part)) { \
put_unaligned_le##bits(val, kaddr + oil); \
--
2.49.0