From 7c1742631e3b67a1aa07a4c78cd1caccc2e32d44 Mon Sep 17 00:00:00 2001 From: Al Viro Date: Tue, 26 Aug 2025 17:27:47 -0400 Subject: [PATCH] alloc_fdtable(): change calling conventions. [ Upstream commit 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 Signed-off-by: Al Viro Signed-off-by: Sasha Levin Signed-off-by: Ulrich Hecht --- fs/file.c | 66 +++++++++++++++++++++++++++++-------------------------- 1 file changed, 35 insertions(+), 31 deletions(-) diff --git a/fs/file.c b/fs/file.c index f92496217d95..8916578b6385 100644 --- a/fs/file.c +++ b/fs/file.c @@ -78,9 +78,17 @@ static void copy_fdtable(struct fdtable *nfdt, struct fdtable *ofdt) copy_fd_bitmaps(nfdt, ofdt, fdt_words(ofdt)); } -static struct fdtable * alloc_fdtable(unsigned int nr) +/* + * Note how the fdtable bitmap allocations very much have to be a multiple of + * BITS_PER_LONG. This is not only because we walk those things in chunks of + * '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". + */ +static struct fdtable *alloc_fdtable(unsigned int slots_wanted) { struct fdtable *fdt; + unsigned int nr; void *data; /* @@ -88,21 +96,32 @@ 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 *)); + 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() @@ -146,7 +165,7 @@ out_arr: out_fdt: kfree(fdt); out: - return NULL; + return ERR_PTR(-ENOMEM); } /* @@ -163,7 +182,7 @@ 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. @@ -172,16 +191,8 @@ static int expand_fdtable(struct files_struct *files, unsigned int nr) synchronize_sched(); 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); @@ -318,16 +329,9 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp) if (new_fdt != &newf->fdtab) __free_fdtable(new_fdt); - new_fdt = alloc_fdtable(open_files - 1); - if (!new_fdt) { - *errorp = -ENOMEM; - goto out_release; - } - - /* beyond sysctl_nr_open; nothing to do */ - if (unlikely(new_fdt->max_fds < open_files)) { - __free_fdtable(new_fdt); - *errorp = -EMFILE; + new_fdt = alloc_fdtable(open_files); + if (IS_ERR(new_fdt)) { + *errorp = PTR_ERR(new_fdt); goto out_release; }