detect-virt: do not try to read all of /proc/cpuinfo

Quoting https://github.com/systemd/systemd/issues/10074:
> detect_vm_uml() reads /proc/cpuinfo with read_full_file()
> read_full_file() has a file max limit size of READ_FULL_BYTES_MAX=(4U*1024U*1024U)
> Unfortunately, the size of my /proc/cpuinfo is bigger, approximately:
> echo $(( 4* $(cat /proc/cpuinfo | wc -c)))
> 9918072
> This causes read_full_file() to fail and the Condition test fallout.

Let's just read line by line until we find an intersting line. This also
helps if not running under UML, because we avoid reading as much data.

(cherry picked from commit 6058516a14ada1748313af6783f5b4e7e3006654)

1 files changed, 28 insertions, 10 deletions

diff --git a/src/basic/virt.c b/src/basic/virt.c
index ac42947a4..94c791ca3 100644
--- a/src/basic/virt.c
+++ b/src/basic/virt.c
@@ -11,6 +11,7 @@
 
 #include "alloc-util.h"
 #include "dirent-util.h"
+//#include "def.h"
 #include "env-util.h"
 #include "fd-util.h"
 #include "fileio.h"
@@ -260,21 +261,38 @@ static int detect_vm_hypervisor(void) {
 }
 
 static int detect_vm_uml(void) {
-        _cleanup_free_ char *cpuinfo_contents = NULL;
+        _cleanup_fclose_ FILE *f = NULL;
         int r;
 
         /* Detect User-Mode Linux by reading /proc/cpuinfo */
-        r = read_full_file("/proc/cpuinfo", &cpuinfo_contents, NULL);
-        if (r == -ENOENT) {
-                log_debug("/proc/cpuinfo not found, assuming no UML virtualization.");
-                return VIRTUALIZATION_NONE;
+        f = fopen("/proc/cpuinfo", "re");
+        if (!f) {
+                if (errno == ENOENT) {
+                        log_debug("/proc/cpuinfo not found, assuming no UML virtualization.");
+                        return VIRTUALIZATION_NONE;
+                }
+                return -errno;
         }
-        if (r < 0)
-                return r;
 
-        if (strstr(cpuinfo_contents, "\nvendor_id\t: User Mode Linux\n")) {
-                log_debug("UML virtualization found in /proc/cpuinfo");
-                return VIRTUALIZATION_UML;
+        for (;;) {
+                _cleanup_free_ char *line = NULL;
+                const char *t;
+
+                r = read_line(f, LONG_LINE_MAX, &line);
+                if (r < 0)
+                        return r;
+                if (r == 0)
+                        break;
+
+                t = startswith(line, "vendor_id\t: ");
+                if (t) {
+                        if (startswith(t, "User Mode Linux")) {
+                                log_debug("UML virtualization found in /proc/cpuinfo");
+                                return VIRTUALIZATION_UML;
+                        }
+
+                        break;
+                }
         }
 
         log_debug("UML virtualization not found in /proc/cpuinfo.");