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Reduce disk write load in /usr/libexec/save-entropy.

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Before this commit, the save-entropy script rotates entropy files
like logs. This involves creating a new file that holds the entropy
and renaming of all existing entropy files. However, the entropy
data do not really need to be kept in a particular order, and
replacing the oldest file is sufficient.

This commit replaces the rotation with a scan in the
[1..entropy_save_num] space that finds the first empty slot, or
the slot of the oldest file, and writes entropy into that slot.

This also fixes an issue that prevents save-entropy from saving
any entropy when there is one non-regular file in any slot as a
side effect.

Based on an earlier patch from peterj@.

PR:		134225
Reported by:	peterj
Reviewed by:	csprng (cem, markm)
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D22612
This commit is contained in:
Xin LI 2019-11-30 20:06:53 +00:00
parent e0a1a1e6cb
commit 46413cedf7
1 changed files with 51 additions and 14 deletions
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65
libexec/save-entropy/save-entropy.sh
View File

@ -71,26 +71,63 @@ cd "${entropy_dir}" || {
for f in saved-entropy.*; do
case "${f}" in saved-entropy.\*) continue ;; esac # No files match
[ ${f#saved-entropy\.} -ge ${entropy_save_num} ] && unlink ${f}
[ ${f#saved-entropy\.} -gt ${entropy_save_num} ] && unlink ${f}
done
umask 377
umask 177
n=$(( ${entropy_save_num} - 1 ))
while [ ${n} -ge 1 ]; do
if [ -f "saved-entropy.${n}" ]; then
mv "saved-entropy.${n}" "saved-entropy.$(( ${n} + 1 ))"
elif [ -e "saved-entropy.${n}" -o -L "saved-entropy.${n}" ]; then
# Scan slots [1..$entropy_save_num), picking an empty slot or the oldest
# existing file if no empty slot was available.
#
# 1. Find out the first regular file or empty slot (and its serial number)
#
n=1
while [ ${n} -le ${entropy_save_num} ]; do
save_file="saved-entropy.${n}"
if [ ! -e "${save_file}" -o -f "${save_file}" ]; then
break
else
logger -is -t "$0" \
"${entropy_dir}/saved-entropy.${n}" is not a regular file, and so \
it will not be rotated. Entropy file rotation is aborted.
exit 1
"${save_file}" is not a regular file, skipped.
fi
n=$(( ${n} - 1 ))
n=$(( ${n} + 1 ))
done
#
# 2. Start from (serial number + 1), and check if the slot is empty
# or is an older regular file, update save_file pointer in either
# case, and break early if we found an empty slot.
#
if [ -f ${save_file} ]; then
n=$(( ${n} + 1 ))
while [ ${n} -le ${entropy_save_num} ]; do
next_file=saved-entropy.${n}
if [ -f "${next_file}" ]; then
[ "${next_file}" -ot "${save_file}" ] && \
save_file="${next_file}"
elif [ ! -e "${next_file}" ]; then
save_file="${next_file}"
break
else
logger -is -t "$0" \
"${next_file}" is not a regular file, skipped.
fi
n=$(( ${n} + 1 ))
done
fi
#
# 3. Check if the pointer we have in hand is really a regular file or
# an empty slot, and bail out as that means there is no available slot.
#
if [ -e "${save_file}" -a ! -f "${save_file}" ]; then
logger -is -t "$0" \
No available slot in "${entropy_dir}", save entropy is aborted.
exit 1
fi
dd if=/dev/random of=saved-entropy.1 bs=${entropy_save_sz} count=1 2>/dev/null
chflags nodump saved-entropy.1 2>/dev/null || :
fsync saved-entropy.1 "."
# Save entropy to the selected slot.
chmod 600 "${save_file}" 2>/dev/null || :
dd if=/dev/random of="${save_file}" bs=${entropy_save_sz} count=1 2>/dev/null
chflags nodump "${save_file}" 2>/dev/null || :
fsync "${save_file}" "."
exit 0