[Bug 2054390] Re: Refine proc mounts entries traversal

[Chengen Du]

** Description changed:

  [Impact]
- When LDAP automount maps generate a substantial number of entries in /proc/mounts, it has the potential to induce a timeout issue, particularly during the process of iterating through the entries twice in the cryptroot hook.
+ The shell's read builtin iterates through /proc/mounts one byte at a time. This becomes problematic when LDAP automount maps generate a large number of entries in /proc/mounts. It can lead to timeout issues, especially when iterating through the entries twice in the cryptroot hook.
  
  [Fix]
- Utilizing awk to initially filter out pertinent mount points results in a notable reduction in the execution time of the hook.
+ Applying the following upstream commit can resolve this issue.
+ 
+ 95fd4be9b4c6 d/functions: get_mnt_devno(): Speed up execution time on large /proc/mounts.
+     Use awk rather than a `while read; do done` loop here as the /proc/mounts
+     pseudo-file can be many thousands lines long and the shell's `read` builtin
+     traverses it one read(2) at the time which cruelly slows down execution time.
+ 
+     See https://salsa.debian.org/cryptsetup-
+ team/cryptsetup/-/merge_requests/36 .
  
  [Test Plan]
- The patch maintains the existing logic within the hook.
  Our primary objective is to ensure that the output remains consistent when obtaining devnos through iteration.
  1. Execute the /usr/share/initramfs-tools/hooks/cryptroot binary
  2. Confirm that the output does not include the following warning message:
  cryptsetup: WARNING: Couldn't determine root device
  
  [Where problems could occur]
  The patch exclusively modifies the method of extracting information without altering the underlying hook logic.
  It's crucial to note that the successful generation of the crypttab is contingent upon the accuracy of the information provided by the patch.
  Any inaccuracies may impede the crypttab generation process.

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https://bugs.launchpad.net/bugs/2054390

Title:
  Refine proc mounts entries traversal

Status in cryptsetup package in Ubuntu:
  In Progress
Status in cryptsetup source package in Focal:
  In Progress
Status in cryptsetup source package in Jammy:
  In Progress
Status in cryptsetup source package in Mantic:
  In Progress
Status in cryptsetup source package in Noble:
  In Progress

Bug description:
  [Impact]
  The shell's read builtin iterates through /proc/mounts one byte at a time. This becomes problematic when LDAP automount maps generate a large number of entries in /proc/mounts. It can lead to timeout issues, especially when iterating through the entries twice in the cryptroot hook.

  [Fix]
  Applying the following upstream commit can resolve this issue.

  95fd4be9b4c6 d/functions: get_mnt_devno(): Speed up execution time on large /proc/mounts.
      Use awk rather than a `while read; do done` loop here as the /proc/mounts
      pseudo-file can be many thousands lines long and the shell's `read` builtin
      traverses it one read(2) at the time which cruelly slows down execution time.

      See https://salsa.debian.org/cryptsetup-
  team/cryptsetup/-/merge_requests/36 .

  [Test Plan]
  Our primary objective is to ensure that the output remains consistent when obtaining devnos through iteration.
  1. Execute the /usr/share/initramfs-tools/hooks/cryptroot binary
  2. Confirm that the output does not include the following warning message:
  cryptsetup: WARNING: Couldn't determine root device

  [Where problems could occur]
  The patch exclusively modifies the method of extracting information without altering the underlying hook logic.
  It's crucial to note that the successful generation of the crypttab is contingent upon the accuracy of the information provided by the patch.
  Any inaccuracies may impede the crypttab generation process.

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