[PATCH AUTOSEL 6.17] erofs: consolidate z_erofs_extent_lookback()
Sasha Levin
sashal at kernel.org
Tue Oct 28 23:48:02 AEDT 2025
From: Gao Xiang <hsiangkao at linux.alibaba.com>
[ Upstream commit 2a13fc417f493e28bdd368785320dd4c2b3d732e ]
The initial m.delta[0] also needs to be checked against zero.
In addition, also drop the redundant logic that errors out for
lcn == 0 / m.delta[0] == 1 case.
Signed-off-by: Gao Xiang <hsiangkao at linux.alibaba.com>
Signed-off-by: Sasha Levin <sashal at kernel.org>
---
LLM Generated explanations, may be completely bogus:
## BACKPORT RECOMMENDATION: **YES**
## EXTENSIVE ANALYSIS
### 1. Code Changes Analysis
The commit makes two specific changes to `fs/erofs/zmap.c`:
**Change 1 - In `z_erofs_extent_lookback()` function
(fs/erofs/zmap.c:272):**
- **BEFORE**: The check `if (!lookback_distance)` occurred AFTER calling
`z_erofs_load_lcluster_from_disk()` and only within the
`Z_EROFS_LCLUSTER_TYPE_NONHEAD` branch
- **AFTER**: The check `if (!lookback_distance)` is moved to the
beginning of the while loop, BEFORE calling
`z_erofs_load_lcluster_from_disk()`
- **Impact**: This prevents unnecessary disk I/O when
`lookback_distance` is 0, and ensures the initial `m.delta[0]` value
is validated before use
**Change 2 - In `z_erofs_map_blocks_fo()` function
(fs/erofs/zmap.c:435-440):**
- **BEFORE**: Code rejected `lcn == 0` as corrupted filesystem with
`EFSCORRUPTED` error
- **AFTER**: This check is completely removed
- **Impact**: Allows valid `lcn == 0` cases to proceed instead of
incorrectly treating them as corruption
### 2. Semantic Analysis Tools Used
I used the following tools to analyze the commit:
- **git log/show**: To examine commit history, related fixes, and code
changes
- **Read tool**: To examine the current state of `fs/erofs/zmap.c`
(lines 260-290, 400-450)
- **Grep tool**: To find all callers of `z_erofs_extent_lookback` and
`z_erofs_map_blocks_fo`
- **Bash**: To examine commit context, tags, and related fixes
### 3. Key Findings from Analysis
**Caller Analysis:**
- `z_erofs_extent_lookback()` is called from `z_erofs_map_blocks_fo()`
at line 446
- `z_erofs_map_blocks_fo()` is called from:
- `z_erofs_map_blocks_iter()` at lines 699 and 756
- `z_erofs_map_blocks_iter()` is the main entry point for EROFS block
mapping
- Called from `z_erofs_iomap_begin_report()` (fiemap operations) and
`zdata.c` (read operations)
**Impact Scope:**
- Affects ALL compressed file reads in EROFS filesystems
- EROFS is widely used in Android and embedded systems
- The bug path is user-triggerable through normal file read operations
**Related Context:**
I discovered that this commit came immediately after TWO critical bug
fixes by the same author:
1. **e13d315ae077b** (2025-10-17): "erofs: avoid infinite loops due to
corrupted subpage compact indexes"
- Has `Fixes:` tags, `Reported-by: Robert Morris`, bug report link
- Fixes infinite loops from crafted images
- **Already backported to stable** as commit a3a04e4d968b0
2. **a429b76114aac** (2025-10-12): "erofs: fix crafted invalid cases for
encoded extents"
- Has `Fixes:` tags, `Reported-by: Robert Morris`, bug report links
- Fixes system crashes from crafted images
- **Being backported to stable**
### 4. Bug Severity Assessment
**Bug #1: Missing initial lookback_distance check**
- **Severity**: Medium (Correctness + Performance)
- **Symptom**: If initial `m.delta[0]` is 0, code performs unnecessary
disk I/O
- **Consequence**: Performance degradation, potential confusion in error
paths
- **Exploitability**: Low - requires specific filesystem state
**Bug #2: False positive corruption error**
- **Severity**: Medium (Functionality)
- **Symptom**: Valid filesystems with `lcn == 0` are rejected as
corrupted
- **Consequence**: Unable to read certain valid compressed files
- **User Impact**: File read failures with `EFSCORRUPTED` errors
### 5. Backport Justification
**Arguments FOR backporting:**
1. **Fixes real bugs**: This is not just a cleanup - it fixes two actual
bugs:
- Unnecessary disk I/O (performance regression)
- False positive corruption errors (functionality regression)
2. **Related to critical fixes**: Comes in a series with two other
critical bug fixes that ARE being backported. Having partial fixes
backported creates inconsistency.
3. **Low risk**:
- Small, focused change (6 insertions, 14 deletions)
- Simplifies code (removes unnecessary branches)
- No new features or architectural changes
4. **Production impact**: EROFS is actively used in Android and embedded
systems. Users could encounter:
- Performance issues from unnecessary I/O
- Mount/read failures from false positive errors
5. **Author expertise**: Gao Xiang is the EROFS maintainer and clearly
understands the correct behavior. The removed check was indeed
"redundant logic" as stated.
**Arguments AGAINST backporting:**
1. **No explicit stable marking**: Author didn't add `Cc:
stable at vger.kernel.org` or `Fixes:` tag
2. **No reported issues**: Unlike the surrounding commits, no bug
reports or CVEs mentioned
3. **Title suggests cleanup**: "consolidate" could imply refactoring
rather than bug fixing
### 6. Final Recommendation Reasoning
Despite the lack of explicit stable marking, I recommend **YES for
backporting** because:
- The commit message explicitly states a bug: "The initial m.delta[0]
also needs to be checked against zero"
- It fixes user-visible issues (false corruption errors)
- It's part of a series addressing EROFS robustness with
crafted/corrupted filesystems
- The surrounding critical fixes ARE being backported
- Small size and low regression risk
- Improves correctness and performance with no downsides
The commit follows the stable kernel rules:
- ✅ Fixes important bugs (performance + false positives)
- ✅ Small and contained change
- ✅ No new features
- ✅ Low regression risk
- ✅ Affects production systems (Android/embedded)
**Recommendation: Backport to all stable trees that contain the affected
code (likely 6.15+ where encoded extents were introduced).**
fs/erofs/zmap.c | 20 ++++++--------------
1 file changed, 6 insertions(+), 14 deletions(-)
diff --git a/fs/erofs/zmap.c b/fs/erofs/zmap.c
index 798223e6da9ce..7a14902af5d38 100644
--- a/fs/erofs/zmap.c
+++ b/fs/erofs/zmap.c
@@ -268,20 +268,19 @@ static int z_erofs_extent_lookback(struct z_erofs_maprecorder *m,
unsigned long lcn = m->lcn - lookback_distance;
int err;
+ if (!lookback_distance)
+ break;
+
err = z_erofs_load_lcluster_from_disk(m, lcn, false);
if (err)
return err;
-
if (m->type == Z_EROFS_LCLUSTER_TYPE_NONHEAD) {
lookback_distance = m->delta[0];
- if (!lookback_distance)
- break;
continue;
- } else {
- m->headtype = m->type;
- m->map->m_la = (lcn << lclusterbits) | m->clusterofs;
- return 0;
}
+ m->headtype = m->type;
+ m->map->m_la = (lcn << lclusterbits) | m->clusterofs;
+ return 0;
}
erofs_err(sb, "bogus lookback distance %u @ lcn %lu of nid %llu",
lookback_distance, m->lcn, vi->nid);
@@ -431,13 +430,6 @@ static int z_erofs_map_blocks_fo(struct inode *inode,
end = inode->i_size;
} else {
if (m.type != Z_EROFS_LCLUSTER_TYPE_NONHEAD) {
- /* m.lcn should be >= 1 if endoff < m.clusterofs */
- if (!m.lcn) {
- erofs_err(sb, "invalid logical cluster 0 at nid %llu",
- vi->nid);
- err = -EFSCORRUPTED;
- goto unmap_out;
- }
end = (m.lcn << lclusterbits) | m.clusterofs;
map->m_flags |= EROFS_MAP_FULL_MAPPED;
m.delta[0] = 1;
--
2.51.0
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