Subject: Re: bug#70877: guix-daemon fails to copy 4+GB file to store

From: Ludovic Courtès <[email-masked]>
Date: Mon, 13 May 2024 12:10:34 +0200

Hi,

Thanks for the bug report and nice reproducer!

Ricardo Wurmus <[email-masked]> skribis:

> The guix-daemon's libutil/util.cc uses copy_file_range to copy a
> downloaded file into the store.  copy_file_range fails on files larger
> than 4GB with an error like this:
>
>     guix build: error: short write in copy_file_range `15' to `16': No such file or directory
>
> The man page for copy_file_range says that it could return EFBIG when
> the range exceeds the maximum range.  The daemon code does not check any
> limits and will attempt to copy the whole file.
>
> I believe our code ought to check the value of st.size and fall back to
> a boring copy if it exceeds some "reasonable" value.

The goal leading to this error message looks like this:

  copy_file_range(15, NULL, 16, NULL, 4294967297, 0) = 2147479552

… which is precisely 2 GiB - 4 KiB.

Reading the man page, it’s entirely fine: like ‘write’,
‘copy_file_range’ might copy less than asked for, so it’s really a
mistake of mine to assume that short writes can’t happen.  Presumably
there’s an internal limit here we’re reaching that explains why it won’t
copy more than 2 GiB at once.

With the following change, we get:

  newfstatat(15, "", {st_mode=S_IFREG|0644, st_size=4294967297, ...}, AT_EMPTY_PATH) = 0
  copy_file_range(15, NULL, 16, NULL, 4294967297, 0) = 2147479552
  copy_file_range(15, NULL, 16, NULL, 2147487745, 0) = 2147479552
  copy_file_range(15, NULL, 16, NULL, 8193, 0) = 8193
  fchown(16, 30001, 30000)          = 0

Could you confirm that it works for you?

Thanks,
Ludo’.

>From efd9f3383756df9959651125c0f2e2e769630851 Mon Sep 17 00:00:00 2001
Message-ID: <[email-masked]>
From: =?UTF-8?q?Ludovic=20Court=C3=A8s?= <[email-masked]>
Date: Mon, 13 May 2024 12:02:30 +0200
Subject: [PATCH] =?UTF-8?q?daemon:=20Loop=20over=20=E2=80=98copy=5Ffile=5F?=
 =?UTF-8?q?range=E2=80=99=20upon=20short=20writes.?=
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Fixes <https://issues.guix.gnu.org/70877>.

* nix/libutil/util.cc (copyFile): Loop over ‘copy_file_range’ instead of
throwing upon short write.

Reported-by: Ricardo Wurmus <[email-masked]>
Change-Id: Id7b8a65ea59006c2d91bc23732309a68665b9ca0
---
 nix/libutil/util.cc | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/nix/libutil/util.cc b/nix/libutil/util.cc
index 578d6572934..3206dea11b1 100644
--- a/nix/libutil/util.cc
+++ b/nix/libutil/util.cc
@@ -397,9 +397,14 @@ static void copyFile(int sourceFd, int destinationFd)
     } else {
 	if (result < 0)
 	    throw SysError(format("copy_file_range `%1%' to `%2%'") % sourceFd % destinationFd);
-	if (result < st.st_size)
-	    throw SysError(format("short write in copy_file_range `%1%' to `%2%'")
-			   % sourceFd % destinationFd);
+
+	/* If 'copy_file_range' copied less than requested, try again.  */
+	for (ssize_t copied = result; copied < st.st_size; copied += result) {
+	    result = copy_file_range(sourceFd, NULL, destinationFd, NULL,
+				     st.st_size - copied, 0);
+	    if (result < 0)
+		throw SysError(format("copy_file_range `%1%' to `%2%'") % sourceFd % destinationFd);
+	}
     }
 }

base-commit: 89cd778f6a45cd9b43a4dc1f236dcd0a87af955c
-- 
2.41.0