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From: Al Viro <viro@zeniv.linux.org.uk>
Date: Tue, 6 Aug 2024 22:14:07 -0400
Subject: alloc_fdtable(): change calling conventions.
Origin: https://git.kernel.org/linus/1d3b4bec3ce55e0c46cdce7d0402dbd6b4af3a3d
First of all, tell it how many slots do we want, not which slot
is wanted. It makes one caller (dup_fd()) more straightforward
and doesn't harm another (expand_fdtable()).
Furthermore, make it return ERR_PTR() on failure rather than
returning NULL. Simplifies the callers.
Simplify the size calculation, while we are at it - note that we
always have slots_wanted greater than BITS_PER_LONG. What the
rules boil down to is
* use the smallest power of two large enough to give us
that many slots
* on 32bit skip 64 and 128 - the minimal capacity we want
there is 256 slots (i.e. 1Kb fd array).
* on 64bit don't skip anything, the minimal capacity is
128 - and we'll never be asked for 64 or less. 128 slots means
1Kb fd array, again.
* on 128bit, if that ever happens, don't skip anything -
we'll never be asked for 128 or less, so the fd array allocation
will be at least 2Kb.
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
fs/file.c | 75 +++++++++++++++++++++----------------------------------
1 file changed, 29 insertions(+), 46 deletions(-)
diff --git a/fs/file.c b/fs/file.c
index 4579c3296498..bfc9eb9e7229 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -90,18 +90,11 @@ static void copy_fdtable(struct fdtable *nfdt, struct fdtable *ofdt)
* 'unsigned long' in some places, but simply because that is how the Linux
* kernel bitmaps are defined to work: they are not "bits in an array of bytes",
* they are very much "bits in an array of unsigned long".
- *
- * The ALIGN(nr, BITS_PER_LONG) here is for clarity: since we just multiplied
- * by that "1024/sizeof(ptr)" before, we already know there are sufficient
- * clear low bits. Clang seems to realize that, gcc ends up being confused.
- *
- * On a 128-bit machine, the ALIGN() would actually matter. In the meantime,
- * let's consider it documentation (and maybe a test-case for gcc to improve
- * its code generation ;)
*/
-static struct fdtable * alloc_fdtable(unsigned int nr)
+static struct fdtable *alloc_fdtable(unsigned int slots_wanted)
{
struct fdtable *fdt;
+ unsigned int nr;
void *data;
/*
@@ -109,22 +102,32 @@ static struct fdtable * alloc_fdtable(unsigned int nr)
* Allocation steps are keyed to the size of the fdarray, since it
* grows far faster than any of the other dynamic data. We try to fit
* the fdarray into comfortable page-tuned chunks: starting at 1024B
- * and growing in powers of two from there on.
+ * and growing in powers of two from there on. Since we called only
+ * with slots_wanted > BITS_PER_LONG (embedded instance in files->fdtab
+ * already gives BITS_PER_LONG slots), the above boils down to
+ * 1. use the smallest power of two large enough to give us that many
+ * slots.
+ * 2. on 32bit skip 64 and 128 - the minimal capacity we want there is
+ * 256 slots (i.e. 1Kb fd array).
+ * 3. on 64bit don't skip anything, 1Kb fd array means 128 slots there
+ * and we are never going to be asked for 64 or less.
*/
- nr /= (1024 / sizeof(struct file *));
- nr = roundup_pow_of_two(nr + 1);
- nr *= (1024 / sizeof(struct file *));
- nr = ALIGN(nr, BITS_PER_LONG);
+ if (IS_ENABLED(CONFIG_32BIT) && slots_wanted < 256)
+ nr = 256;
+ else
+ nr = roundup_pow_of_two(slots_wanted);
/*
* Note that this can drive nr *below* what we had passed if sysctl_nr_open
- * had been set lower between the check in expand_files() and here. Deal
- * with that in caller, it's cheaper that way.
+ * had been set lower between the check in expand_files() and here.
*
* We make sure that nr remains a multiple of BITS_PER_LONG - otherwise
* bitmaps handling below becomes unpleasant, to put it mildly...
*/
- if (unlikely(nr > sysctl_nr_open))
- nr = ((sysctl_nr_open - 1) | (BITS_PER_LONG - 1)) + 1;
+ if (unlikely(nr > sysctl_nr_open)) {
+ nr = round_down(sysctl_nr_open, BITS_PER_LONG);
+ if (nr < slots_wanted)
+ return ERR_PTR(-EMFILE);
+ }
/*
* Check if the allocation size would exceed INT_MAX. kvmalloc_array()
@@ -168,7 +171,7 @@ static struct fdtable * alloc_fdtable(unsigned int nr)
out_fdt:
kfree(fdt);
out:
- return NULL;
+ return ERR_PTR(-ENOMEM);
}
/*
@@ -185,7 +188,7 @@ static int expand_fdtable(struct files_struct *files, unsigned int nr)
struct fdtable *new_fdt, *cur_fdt;
spin_unlock(&files->file_lock);
- new_fdt = alloc_fdtable(nr);
+ new_fdt = alloc_fdtable(nr + 1);
/* make sure all fd_install() have seen resize_in_progress
* or have finished their rcu_read_lock_sched() section.
@@ -194,16 +197,8 @@ static int expand_fdtable(struct files_struct *files, unsigned int nr)
synchronize_rcu();
spin_lock(&files->file_lock);
- if (!new_fdt)
- return -ENOMEM;
- /*
- * extremely unlikely race - sysctl_nr_open decreased between the check in
- * caller and alloc_fdtable(). Cheaper to catch it here...
- */
- if (unlikely(new_fdt->max_fds <= nr)) {
- __free_fdtable(new_fdt);
- return -EMFILE;
- }
+ if (IS_ERR(new_fdt))
+ return PTR_ERR(new_fdt);
cur_fdt = files_fdtable(files);
BUG_ON(nr < cur_fdt->max_fds);
copy_fdtable(new_fdt, cur_fdt);
@@ -322,7 +317,6 @@ struct files_struct *dup_fd(struct files_struct *oldf, struct fd_range *punch_ho
struct file **old_fds, **new_fds;
unsigned int open_files, i;
struct fdtable *old_fdt, *new_fdt;
- int error;
newf = kmem_cache_alloc(files_cachep, GFP_KERNEL);
if (!newf)
@@ -354,17 +348,10 @@ struct files_struct *dup_fd(struct files_struct *oldf, struct fd_range *punch_ho
if (new_fdt != &newf->fdtab)
__free_fdtable(new_fdt);
- new_fdt = alloc_fdtable(open_files - 1);
- if (!new_fdt) {
- error = -ENOMEM;
- goto out_release;
- }
-
- /* beyond sysctl_nr_open; nothing to do */
- if (unlikely(new_fdt->max_fds < open_files)) {
- __free_fdtable(new_fdt);
- error = -EMFILE;
- goto out_release;
+ new_fdt = alloc_fdtable(open_files);
+ if (IS_ERR(new_fdt)) {
+ kmem_cache_free(files_cachep, newf);
+ return ERR_CAST(new_fdt);
}
/*
@@ -413,10 +400,6 @@ struct files_struct *dup_fd(struct files_struct *oldf, struct fd_range *punch_ho
rcu_assign_pointer(newf->fdt, new_fdt);
return newf;
-
-out_release:
- kmem_cache_free(files_cachep, newf);
- return ERR_PTR(error);
}
static struct fdtable *close_files(struct files_struct * files)
--
2.50.1
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