blk-throttle: fix divide-by-zero on legacy iops limit of 0
Subsystems (3)
PATCH v3 1/1 blk-throttle: fix divide-by-zero on legacy iops limit of 0
From: Tao Cui <[email protected]>
Writing a multiple of 2^32 (e.g. 4294967296) to a legacy cgroup v1 throttle iops file (blkio.throttle.{read,write}_iops_device) silently truncates to 0: tg_set_conf() stores the sscanf-parsed u64 value into an unsigned int field with no clamping. The cgroup v2 path, tg_set_limit(), already clamps the same kind of value with min_t(u64, val, UINT_MAX), but the legacy path never did. Note that the "!v -> U64_MAX" mapping only catches an explicit zero and does not catch a value that truncates to zero.
With iops stored as 0, tg_update_has_rules() sets has_rules_iops[] and the next IO reaches tg_within_iops_limit(), which computes
jiffy_wait = max(jiffy_wait, HZ / iops_limit + 1);
triggering a divide-by-zero oops. The unclamped write in tg_set_conf() is long-standing, but it only became a crash once the HZ / iops_limit divide was added.
Fix it in two places:
* tg_set_conf(): clamp the value to UINT_MAX, consistent with tg_set_limit(). This closes the truncation root cause (and the general silent truncation for any value above UINT_MAX).
* tg_dispatch_iops_time(): treat iops_limit == 0 as unlimited so the divide in tg_within_iops_limit() is never reached, defending against any future path that could produce a zero limit.
---
Changes in v3:
- Drop the (u64) cast on UINT_MAX: the kernel's type-checked min() accepts two unsigned types of different width (both >= 4 bytes), so min(v, UINT_MAX) compiles clean. (David Laight)
- Add a Fixes: tag pointing at the commit that introduced the HZ / iops_limit divide, which is also where the oops became reachable.
Changes in v2:
- Use a "void *field" local for the config write so the assignment reads *(u64 *)field / *(unsigned int *)field instead of the (type *)((void *)tg + of_cft(of)->private) casts.
- Use min(v, UINT_MAX) instead of min_t(u64, v, UINT_MAX).
--- block/blk-throttle.c | 13 ++++++++++--- 1 file changed, 10 insertions(+), 3 deletions(-)
block/blk-throttle.c +10 -3
@@ -883,7 +883,12 @@ static unsigned long tg_dispatch_iops_time(struct throtl_grp *tg, struct bio *biu32 iops_limit =tg_iops_limit (tg, rw);unsigned long iops_wait; -if (iops_limit ==UINT_MAX || tg->flags &THROTL_TG_CANCELING ) +/* +* iops_limit == 0 is not a valid limit. Treat it as unlimited so we +* never reach the HZ / iops_limit divide in tg_within_iops_limit(). +*/ +if (iops_limit ==UINT_MAX || iops_limit ==0 || + tg->flags &THROTL_TG_CANCELING )return 0 ;tg_update_slice (tg, rw);
@@ -1383,10 +1388,12 @@ static ssize_t tg_set_conf(struct kernfs_open_file *of, tg =blkg_to_tg (ctx.blkg);tg_update_carryover (tg); +void *field = (void *)tg +of_cft (of)->private; +if (is_u64) - *(u64 *)((void *)tg +of_cft (of)->private) = v; + *(u64 *)field = v;else - *(unsigned int *)((void *)tg +of_cft (of)->private) = v; + *(unsigned int *)field =min (v,UINT_MAX );tg_conf_updated (tg, false); ret =0 ;
Signature · 1 line
2.43.0
Hi,
Quoted 26 lines · Tao Cui
From: Tao Cui <[email protected]>
Writing a multiple of 2^32 (e.g. 4294967296) to a legacy cgroup v1 throttle iops file (blkio.throttle.{read,write}_iops_device) silently truncates to 0:
tg_set_conf()stores the sscanf-parsed u64 value into an unsigned int field with no clamping. The cgroup v2 path,tg_set_limit(), already clamps the same kind of value withmin_t(u64, val, UINT_MAX), but the legacy path never did. Note that the "!v ->U64_MAX" mapping only catches an explicit zero and does not catch a value that truncates to zero.With iops stored as 0,
tg_update_has_rules()sets has_rules_iops[] and the next IO reachestg_within_iops_limit(), which computesjiffy_wait = max(jiffy_wait, HZ / iops_limit + 1);triggering a divide-by-zero oops. The unclamped write in
tg_set_conf()is long-standing, but it only became a crash once the HZ /iops_limitdivide was added.Fix it in two places:
*
tg_set_conf(): clamp the value toUINT_MAX, consistent withtg_set_limit(). This closes the truncation root cause (and the general silent truncation for any value aboveUINT_MAX).
Quoted 5 lines
*
tg_dispatch_iops_time(): treatiops_limit== 0 as unlimited so the divide intg_within_iops_limit()is never reached, defending against any future path that could produce a zero limit.Fixes 1beabab88ece ("blk-throttle: fix lower control under super low iops limit")
I think the root cause is that iops_limit set to 0, and what's the behavior of throttle with iops=0? Since calculate_io_allowed() will always return 0, it's likely that no IO can be issued.
Quoted 25 lines
Signed-off-by Tao Cui <[email protected]>---
Changes in v3:
- Drop the (u64) cast onUINT_MAX: the kernel's type-checkedmin()accepts two unsigned types of different width (both >= 4 bytes), somin(v, UINT_MAX)compiles clean. (David Laight)
- Add a Fixes: tag pointing at the commit that introduced the HZ /iops_limitdivide, which is also where the oops became reachable.Changes in v2:
- Use a "void *field" local for the config write so the assignment reads *(u64 *)field / *(unsigned int *)field instead of the (type *)((void *)tg + of_cft(of)->private) casts.
- Usemin(v, UINT_MAX)instead ofmin_t(u64, v, UINT_MAX).--- block/blk-throttle.c | 13 ++++++++++--- 1 file changed, 10 insertions(+), 3 deletions(-)block/blk-throttle.c +0
-0@@ -883,7 +883,12 @@ static unsigned long tg_dispatch_iops_time(struct throtl_grp *tg, struct bio *biu32 iops_limit =tg_iops_limit (tg, rw);
block/blk-throttle.c static unsigned long tg_dispatch_iops_time(struct throtl_grp *tg, struct bio *bi · line 892
Quoted 10 lines
block/blk-throttle.c +6
-1unsigned long iops_wait; -if (iops_limit ==UINT_MAX || tg->flags &THROTL_TG_CANCELING ) +/* +* iops_limit == 0 is not a valid limit. Treat it as unlimited so we +* never reach the HZ / iops_limit divide in tg_within_iops_limit(). +*/ +if (iops_limit ==UINT_MAX || iops_limit ==0 || + tg->flags &THROTL_TG_CANCELING )return 0 ;
Is the above still needed with the tg_set_conf() changes? 0 is not a valid limit, and it should not exist in the first place, so I don't like the above new checking.
Quoted 17 lines
block/blk-throttle.c +4
-2tg_update_slice (tg, rw);@@ -1383,10 +1388,12 @@ static ssize_t tg_set_conf(struct kernfs_open_file *of, tg =blkg_to_tg (ctx.blkg);tg_update_carryover (tg); +void *field = (void *)tg +of_cft (of)->private; +if (is_u64) - *(u64 *)((void *)tg +of_cft (of)->private) = v; + *(u64 *)field = v;else - *(unsigned int *)((void *)tg +of_cft (of)->private) = v; + *(unsigned int *)field =min (v,UINT_MAX );tg_conf_updated (tg, false); ret =0 ;
Signature · 2 lines
Thanks, Kuai
Quoted 35 lines · yu kuai
Hi,
Quoted 31 lines · Tao Cui
From: Tao Cui <[email protected]>
Writing a multiple of 2^32 (e.g. 4294967296) to a legacy cgroup v1 throttle iops file (blkio.throttle.{read,write}_iops_device) silently truncates to 0:
tg_set_conf()stores the sscanf-parsed u64 value into an unsigned int field with no clamping. The cgroup v2 path,tg_set_limit(), already clamps the same kind of value withmin_t(u64, val, UINT_MAX), but the legacy path never did. Note that the "!v ->U64_MAX" mapping only catches an explicit zero and does not catch a value that truncates to zero.With iops stored as 0,
tg_update_has_rules()sets has_rules_iops[] and the next IO reachestg_within_iops_limit(), which computesjiffy_wait = max(jiffy_wait, HZ / iops_limit + 1);triggering a divide-by-zero oops. The unclamped write in
tg_set_conf()is long-standing, but it only became a crash once the HZ /iops_limitdivide was added.Fix it in two places:
*
tg_set_conf(): clamp the value toUINT_MAX, consistent withtg_set_limit(). This closes the truncation root cause (and the general silent truncation for any value aboveUINT_MAX).*
tg_dispatch_iops_time(): treatiops_limit== 0 as unlimited so the divide intg_within_iops_limit()is never reached, defending against any future path that could produce a zero limit.Fixes 1beabab88ece ("blk-throttle: fix lower control under super low iops limit")
Hi Kuai,
Thanks for the review, and you're right on both points.
Quoted 3 lines
I think the root cause is that
iops_limitset to 0, and what's the behavior of throttle with iops=0? Sincecalculate_io_allowed()will always return 0, it's likely that no IO can be issued.
On the root cause: yes, iops=0 is the real root cause and it predates 1beabab88ece. With iops_limit = 0, calculate_io_allowed() returns 0 (0 * jiffy_elapsed), so no IO can be issued at all; 1beabab88ece only turned that pre-existing iops=0 hang into a divide-by-zero via the new HZ / iops_limit. Attributing it to 1beabab88ece was therefore wrong -- my apologies. I'll drop the Fixes: tag in v4.
Quoted 28 lines
Quoted 28 lines
Signed-off-by Tao Cui <[email protected]>---
Changes in v3:
- Drop the (u64) cast onUINT_MAX: the kernel's type-checkedmin()accepts two unsigned types of different width (both >= 4 bytes), somin(v, UINT_MAX)compiles clean. (David Laight)
- Add a Fixes: tag pointing at the commit that introduced the HZ /iops_limitdivide, which is also where the oops became reachable.Changes in v2:
- Use a "void *field" local for the config write so the assignment reads *(u64 *)field / *(unsigned int *)field instead of the (type *)((void *)tg + of_cft(of)->private) casts.
- Usemin(v, UINT_MAX)instead ofmin_t(u64, v, UINT_MAX).--- block/blk-throttle.c | 13 ++++++++++--- 1 file changed, 10 insertions(+), 3 deletions(-)block/blk-throttle.c +0
-0@@ -883,7 +883,12 @@ static unsigned long tg_dispatch_iops_time(struct throtl_grp *tg, struct bio *biu32 iops_limit =tg_iops_limit (tg, rw);unsigned long iops_wait;
Quoted 11 lines
block/blk-throttle.c static unsigned long tg_dispatch_iops_time(struct throtl_grp *tg, struct bio *bi · line 892
Quoted 8 lines
block/blk-throttle.c +6
-1-if (iops_limit ==UINT_MAX || tg->flags &THROTL_TG_CANCELING ) +/* +* iops_limit == 0 is not a valid limit. Treat it as unlimited so we +* never reach the HZ / iops_limit divide in tg_within_iops_limit(). +*/ +if (iops_limit ==UINT_MAX || iops_limit ==0 || + tg->flags &THROTL_TG_CANCELING )return 0 ;Is the above still needed with the
tg_set_conf()changes? 0 is not a valid limit, and it should not exist in the first place, so I don't like the above new checking.
On the iops_limit == 0 check: agreed, dropping it. With the tg_set_conf() clamp in place, 0 can never be stored as a limit (explicit 0 maps to U64_MAX, anything that would truncate to 0 is clamped to UINT_MAX, and the init value is UINT_MAX), so the runtime check only guards an unreachable state. v4 keeps only the tg_set_conf() clamp.
Will send v4 shortly.
Thanks,
Tao
Quoted 19 lines
Quoted 18 lines
block/blk-throttle.c +4
-2tg_update_slice (tg, rw);@@ -1383,10 +1388,12 @@ static ssize_t tg_set_conf(struct kernfs_open_file *of, tg =blkg_to_tg (ctx.blkg);tg_update_carryover (tg); +void *field = (void *)tg +of_cft (of)->private; +if (is_u64) - *(u64 *)((void *)tg +of_cft (of)->private) = v; + *(u64 *)field = v;else - *(unsigned int *)((void *)tg +of_cft (of)->private) = v; + *(unsigned int *)field =min (v,UINT_MAX );tg_conf_updated (tg, false); ret =0 ;