Subject: bug#59382: cp(1) tries to allocate too much memory if filesystem blocksizes are unusual

From: Paul Eggert <[email-masked]>
Date: Sat, 19 Nov 2022 19:50:06 -0800

>> The block size for filesystems can also be quite large (currently, up 
>> to 16M).

It seems ZFS tries to "help" apps by reporting misinformation (namely a 
smaller block size than actually preferred) when the file is small. This 
is unfortunate, since it messes up cp and similar programs that need to 
juggle multiple block sizes. Plus, it messes up any program that assumes 
st_blksize is constant for the life of a file descriptor, which "cp" 
does assume elsewhere.

GNU cp doesn't need ZFS's "help", as it's already smart enough to not 
over-allocate a buffer when the input file is small but its blocksize is 
large. Instead, this "help" from ZFS causes GNU cp to over-allocate 
because it naively trusts the blocksize ZFS that reports.

> The proposed patch attached removes the use of buffer_lcm()
> and just picks the largest st_blksize, which would be 4MiB in your case.
> It also limits the max buffer size to 32MiB in the edge case
> where st_blksize returns a larger value that this.

I suppose this could break cp if st_blksize is not a power of 2, and if 
the file is not a regular file, and reads must be a multiple of the 
block size. POSIX allows such things though I expect nowadays it'd be 
limited to weird devices.

Although we inadvertently removed support for weird devices in 2009 by 
commit 55efc5f3ee485b3e31a91c331f07c89aeccc4e89, and nobody seems to 
care (because people use dd or whatever to deal with weird devices), I 
think it'd be better to limit the fix to regular files. And while we're 
at it we might as well resurrect support for weird devices.

> +#include <assert.h>

No need for this, as static_assert works without <assert.h> in C23, and 
Gnulib's assert-h module support this.

> +/* Set a max constraint to avoid excessive mem usage or type overflow.  */
> +enum { IO_BUFSIZE_MAX = 128 * IO_BUFSIZE };
> +static_assert (IO_BUFSIZE_MAX <= MIN (IDX_MAX, SIZE_MAX) / 2 + 1);

I'm leery of putting in a maximum as low as 16 MiB. Although that's OK 
now (it matches OpenZFS's current maximum), cp in the future will surely 
deal with bigger block sizes. Instead, how about if we stick with GNU's 
"no arbitrary limits" policy and work around the ZFS bug instead?

Something like the attached patch, perhaps?

From 551f3f55180669ab0bfd6c5d9e3e0f38cb035172 Mon Sep 17 00:00:00 2001
From: Paul Eggert <[email-masked]>
Date: Sat, 19 Nov 2022 19:04:36 -0800
Subject: [PATCH] cp: work around ZFS misinformation

Problem reported by Korn Andras (Bug#59382).
* bootstrap.conf (gnulib_modules): Add count-leading-zeros,
which was already an indirect dependency, since ioblksize.h
now uses it directly.
* src/ioblksize.h: Include count-leading-zeros.h.
(io_blksize): Treat impossible blocksizes as IO_BUFSIZE.
When growing a blocksize to IO_BUFSIZE, keep it a multiple of the
stated blocksize.  Work around the ZFS performance bug.
---
 NEWS            |  3 +++
 bootstrap.conf  |  1 +
 src/ioblksize.h | 28 +++++++++++++++++++++++++++-
 3 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/NEWS b/NEWS
index b6b5201e7..9282352c8 100644
--- a/NEWS
+++ b/NEWS
@@ -13,6 +13,9 @@ GNU coreutils NEWS                                    -*- outline -*-
   'cp -rx / /mnt' no longer complains "cannot create directory /mnt/".
   [bug introduced in coreutils-9.1]
 
+  cp, mv, and install no longer use overly large I/O buffers when ZFS
+  misinforms them about IO block sizes.
+
   'mv --backup=simple f d/' no longer mistakenly backs up d/f to f~.
   [bug introduced in coreutils-9.1]
 
diff --git a/bootstrap.conf b/bootstrap.conf
index 8e257a254..f8715068e 100644
--- a/bootstrap.conf
+++ b/bootstrap.conf
@@ -59,6 +59,7 @@ gnulib_modules="
   config-h
   configmake
   copy-file-range
+  count-leading-zeros
   crypto/md5
   crypto/sha1
   crypto/sha256
diff --git a/src/ioblksize.h b/src/ioblksize.h
index 8bd18ba05..aa367aa4e 100644
--- a/src/ioblksize.h
+++ b/src/ioblksize.h
@@ -18,6 +18,7 @@
 
 /* sys/stat.h and minmax.h will already have been included by system.h. */
 #include "idx.h"
+#include "count-leading-zeros.h"
 #include "stat-size.h"

@@ -75,8 +76,33 @@ enum { IO_BUFSIZE = 128 * 1024 };
 static inline idx_t
 io_blksize (struct stat sb)
 {
+  /* Treat impossible blocksizes as if they were IO_BUFSIZE.  */
+  idx_t blocksize = ST_BLKSIZE (sb) <= 0 ? IO_BUFSIZE : ST_BLKSIZE (sb);
+
+  /* Use a blocksize of at least IO_BUFSIZE bytes, keeping it a
+     multiple of the original blocksize.  */
+  blocksize += (IO_BUFSIZE - 1) - (IO_BUFSIZE - 1) % blocksize;
+
+  /* For regular files we can ignore the blocksize if we think we know better.
+     ZFS sometimes understates the blocksize, because it thinks
+     apps stupidly allocate a block that large even for small files.
+     This misinformation can cause coreutils to use wrong-sized blocks.
+     Work around some of the performance bug by substituting the next
+     power of two when the reported blocksize is not a power of two.  */
+  if (S_ISREG (sb.st_mode)
+      && blocksize & (blocksize - 1))
+    {
+      int leading_zeros = count_leading_zeros_ll (blocksize);
+      if (IDX_MAX < ULLONG_MAX || leading_zeros)
+        {
+          unsigned long long power = 1ull << (ULLONG_WIDTH - leading_zeros);
+          if (power <= IDX_MAX)
+            blocksize = power;
+        }
+    }
+
   /* Don’t go above the largest power of two that fits in idx_t and size_t,
      as that is asking for trouble.  */
   return MIN (MIN (IDX_MAX, SIZE_MAX) / 2 + 1,
-              MAX (IO_BUFSIZE, ST_BLKSIZE (sb)));
+              blocksize);
 }
-- 
2.37.2