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Subject[PATCH] prctl_set_mm: downgrade mmap_sem to read lock
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I learnt, it's, alas, too late to drop the non PRCTL_SET_MM_MAP calls
[1], so at least downgrade the write acquisition of mmap_sem as in the
patch below (that should be stacked on the previous one or squashed).

Cyrill, you mentioned lock changes in [1] but the link seems empty. Is
it supposed to be [2]? That could be an alternative to this patch after
some refreshments and clarifications.


[1] https://lore.kernel.org/lkml/20190417165632.GC3040@uranus.lan/
[2] https://lore.kernel.org/lkml/20180507075606.870903028@gmail.com/

========

Since commit 88aa7cc688d4 ("mm: introduce arg_lock to protect
arg_start|end and env_start|end in mm_struct") we use arg_lock for
boundaries modifications. Synchronize prctl_set_mm with this lock and
keep mmap_sem for reading only (analogous to what we already do in
prctl_set_mm_map).

Also, save few cycles by looking up VMA only after performing basic
arguments validation.

Signed-off-by: Michal Koutný <mkoutny@suse.com>
---
kernel/sys.c | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index 12df0e5434b8..bbce0f26d707 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2125,8 +2125,12 @@ static int prctl_set_mm(int opt, unsigned long addr,

error = -EINVAL;

- down_write(&mm->mmap_sem);
- vma = find_vma(mm, addr);
+ /*
+ * arg_lock protects concurent updates of arg boundaries, we need mmap_sem for
+ * a) concurrent sys_brk, b) finding VMA for addr validation.
+ */
+ down_read(&mm->mmap_sem);
+ spin_lock(&mm->arg_lock);

prctl_map.start_code = mm->start_code;
prctl_map.end_code = mm->end_code;
@@ -2185,6 +2189,7 @@ static int prctl_set_mm(int opt, unsigned long addr,
if (error)
goto out;

+ vma = find_vma(mm, addr);
switch (opt) {
/*
* If command line arguments and environment
@@ -2218,7 +2223,8 @@ static int prctl_set_mm(int opt, unsigned long addr,

error = 0;
out:
- up_write(&mm->mmap_sem);
+ spin_unlock(&mm->arg_lock);
+ up_read(&mm->mmap_sem);
return error;
}

--
2.16.4
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