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199 lines
9.6 KiB
199 lines
9.6 KiB
1 year ago
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From 7955f053c498a70616ac6c4d57c0fd47dfa8e5ac Mon Sep 17 00:00:00 2001
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1 year ago
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From: =?UTF-8?q?Zbigniew=20J=C4=99drzejewski-Szmek?= <zbyszek@in.waw.pl>
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Date: Wed, 7 Oct 2020 11:15:05 +0200
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Subject: [PATCH] pager: make pager secure when under euid is changed or
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explicitly requested
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The variable is renamed to SYSTEMD_PAGERSECURE (because it's not just about
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less now), and we automatically enable secure mode in certain cases, but not
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otherwise.
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This approach is more nuanced, but should provide a better experience for
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users:
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- Previusly we would set LESSSECURE=1 and trust the pager to make use of
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it. But this has an effect only on less. We need to not start pagers which
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are insecure when in secure mode. In particular more is like that and is a
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very popular pager.
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- We don't enable secure mode always, which means that those other pagers can
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reasonably used.
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- We do the right thing by default, but the user has ultimate control by
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setting SYSTEMD_PAGERSECURE.
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Fixes #5666.
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v2:
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- also check $PKEXEC_UID
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v3:
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- use 'sd_pid_get_owner_uid() != geteuid()' as the condition
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(cherry picked from commit 0a42426d797406b4b01a0d9c13bb759c2629d108)
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1 year ago
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Resolves: #2175624
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1 year ago
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---
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man/less-variables.xml | 28 ++++++++++++++---
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meson.build | 3 +-
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src/basic/pager.c | 69 +++++++++++++++++++++++++++---------------
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3 files changed, 69 insertions(+), 31 deletions(-)
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diff --git a/man/less-variables.xml b/man/less-variables.xml
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index 9dad4247da..5f3a53c8dd 100644
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--- a/man/less-variables.xml
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+++ b/man/less-variables.xml
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@@ -37,12 +37,30 @@
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</varlistentry>
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<varlistentry id='lesssecure'>
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- <term><varname>$SYSTEMD_LESSSECURE</varname></term>
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+ <term><varname>$SYSTEMD_PAGERSECURE</varname></term>
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- <listitem><para>Takes a boolean argument. Overrides the <varname>$LESSSECURE</varname> environment
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- variable when invoking the pager, which controls the "secure" mode of less (which disables commands
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- such as <literal>|</literal> which allow to easily shell out to external command lines). By default
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- less secure mode is enabled, with this setting it may be disabled.</para></listitem>
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+ <listitem><para>Takes a boolean argument. When true, the "secure" mode of the pager is enabled; if
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+ false, disabled. If <varname>$SYSTEMD_PAGERSECURE</varname> is not set at all, secure mode is enabled
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+ if the effective UID is not the same as the owner of the login session, see <citerefentry
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+ project='man-pages'><refentrytitle>geteuid</refentrytitle><manvolnum>2</manvolnum></citerefentry> and
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+ <citerefentry><refentrytitle>sd_pid_get_owner_uid</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
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+ In secure mode, <option>LESSSECURE=1</option> will be set when invoking the pager, and the pager shall
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+ disable commands that open or create new files or start new subprocesses. When
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+ <varname>$SYSTEMD_PAGERSECURE</varname> is not set at all, pagers which are not known to implement
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+ secure mode will not be used. (Currently only
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+ <citerefentry><refentrytitle>less</refentrytitle><manvolnum>1</manvolnum></citerefentry> implements
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+ secure mode.)</para>
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+
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+ <para>Note: when commands are invoked with elevated privileges, for example under <citerefentry
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+ project='man-pages'><refentrytitle>sudo</refentrytitle><manvolnum>8</manvolnum></citerefentry> or
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+ <citerefentry
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+ project='die-net'><refentrytitle>pkexec</refentrytitle><manvolnum>1</manvolnum></citerefentry>, care
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+ must be taken to ensure that unintended interactive features are not enabled. "Secure" mode for the
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+ pager may be enabled automatically as describe above. Setting <varname>SYSTEMD_PAGERSECURE=0</varname>
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+ or not removing it from the inherited environment allows the user to invoke arbitrary commands. Note
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+ that if the <varname>$SYSTEMD_PAGER</varname> or <varname>$PAGER</varname> variables are to be
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+ honoured, <varname>$SYSTEMD_PAGERSECURE</varname> must be set too. It might be reasonable to completly
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+ disable the pager using <option>--no-pager</option> instead.</para></listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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diff --git a/meson.build b/meson.build
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index 673800a1a7..d986dd24ac 100644
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--- a/meson.build
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+++ b/meson.build
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@@ -1467,7 +1467,8 @@ test_dlopen = executable(
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'test-dlopen',
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test_dlopen_c,
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include_directories : includes,
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- link_with : [libbasic],
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+ link_with : [libsystemd_static,
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+ libbasic],
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dependencies : [libdl])
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foreach tuple : [['myhostname', 'ENABLE_NSS_MYHOSTNAME'],
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diff --git a/src/basic/pager.c b/src/basic/pager.c
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index 4efb01c483..c7e101235d 100644
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--- a/src/basic/pager.c
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+++ b/src/basic/pager.c
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@@ -10,6 +10,8 @@
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#include <sys/prctl.h>
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#include <unistd.h>
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+#include "sd-login.h"
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+
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#include "copy.h"
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#include "env-util.h"
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#include "fd-util.h"
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@@ -79,7 +81,7 @@ int pager_open(bool no_pager, bool jump_to_end) {
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if (r < 0)
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return r;
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if (r == 0) {
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- const char* less_opts, *less_charset;
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+ const char* less_opts, *less_charset, *exe;
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/* In the child start the pager */
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@@ -105,39 +107,56 @@ int pager_open(bool no_pager, bool jump_to_end) {
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_exit(EXIT_FAILURE);
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/* People might invoke us from sudo, don't needlessly allow less to be a way to shell out
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- * privileged stuff. */
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- r = getenv_bool("SYSTEMD_LESSSECURE");
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- if (r == 0) { /* Remove env var if off */
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- if (unsetenv("LESSSECURE") < 0) {
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- log_error_errno(errno, "Failed to uset environment variable LESSSECURE: %m");
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- _exit(EXIT_FAILURE);
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- }
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- } else {
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- /* Set env var otherwise */
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+ * privileged stuff. If the user set $SYSTEMD_PAGERSECURE, trust their configuration of the
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+ * pager. If they didn't, use secure mode when under euid is changed. If $SYSTEMD_PAGERSECURE
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+ * wasn't explicitly set, and we autodetect the need for secure mode, only use the pager we
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+ * know to be good. */
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+ int use_secure_mode = getenv_bool("SYSTEMD_PAGERSECURE");
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+ bool trust_pager = use_secure_mode >= 0;
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+ if (use_secure_mode == -ENXIO) {
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+ uid_t uid;
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+
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+ r = sd_pid_get_owner_uid(0, &uid);
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if (r < 0)
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- log_warning_errno(r, "Unable to parse $SYSTEMD_LESSSECURE, ignoring: %m");
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+ log_debug_errno(r, "sd_pid_get_owner_uid() failed, enabling pager secure mode: %m");
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+
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+ use_secure_mode = r < 0 || uid != geteuid();
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+
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+ } else if (use_secure_mode < 0) {
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+ log_warning_errno(use_secure_mode, "Unable to parse $SYSTEMD_PAGERSECURE, assuming true: %m");
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+ use_secure_mode = true;
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+ }
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- if (setenv("LESSSECURE", "1", 1) < 0) {
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- log_error_errno(errno, "Failed to set environment variable LESSSECURE: %m");
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- _exit(EXIT_FAILURE);
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- }
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+ /* We generally always set variables used by less, even if we end up using a different pager.
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+ * They shouldn't hurt in any case, and ideally other pagers would look at them too. */
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+ if (use_secure_mode)
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+ r = setenv("LESSSECURE", "1", 1);
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+ else
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+ r = unsetenv("LESSSECURE");
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+ if (r < 0) {
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+ log_error_errno(errno, "Failed to adjust environment variable LESSSECURE: %m");
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+ _exit(EXIT_FAILURE);
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}
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- if (pager) {
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+ if (trust_pager && pager) { /* The pager config might be set globally, and we cannot
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+ * know if the user adjusted it to be appropriate for the
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+ * secure mode. Thus, start the pager specified through
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+ * envvars only when $SYSTEMD_PAGERSECURE was explicitly set
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+ * as well. */
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execlp(pager, pager, NULL);
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execl("/bin/sh", "sh", "-c", pager, NULL);
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}
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- /* Debian's alternatives command for pagers is
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- * called 'pager'. Note that we do not call
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- * sensible-pagers here, since that is just a
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- * shell script that implements a logic that
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- * is similar to this one anyway, but is
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- * Debian-specific. */
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- execlp("pager", "pager", NULL);
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+ /* Debian's alternatives command for pagers is called 'pager'. Note that we do not call
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+ * sensible-pagers here, since that is just a shell script that implements a logic that is
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+ * similar to this one anyway, but is Debian-specific. */
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+ FOREACH_STRING(exe, "pager", "less", "more") {
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+ /* Only less implements secure mode right now. */
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+ if (use_secure_mode && !streq(exe, "less"))
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+ continue;
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- execlp("less", "less", NULL);
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- execlp("more", "more", NULL);
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+ execlp(exe, exe, NULL);
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+ }
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pager_fallback();
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/* not reached */
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