[PATCH lttng-tools v2] Fix: syscall_table_nb_entry invalid value when no syscalls TPs are defined

[Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>]

v2: merged previous series in one patch since the second patch is a minor fix
and partly addressed in the first patch.

"start" -> starts

--

fscanf on an empty file returns directly without assigning value to
'index' leading to assigning the value of an uninitialized variable to
syscall_table_nb_entry. This can result in memory allocation problems
when listing syscalls on 'lttng list --kernel --syscall'[1][2].

Fixes at the same time an off-by-one problem for the index
value.

The index value returned by fscanf is an index starting at 0.
However, it is later used to count how many elements need to be
allocated, without any extra increment.

It does not cause issues in practice because SYSCALL_TABLE_INIT_SIZE
is nonzero, and because we don't require the table to expand by more
than the double of its size at once (which could happen if we could
have a hole in the syscall table for instance).

It's better to fix this off-by-one for clarity, and in case this code
gets re-used in other contexts that don't have the same memory growth
pattern.

Fixes #1091

[1] https://bugs.lttng.org/issues/1091
[2] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1671063/

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
---
 src/bin/lttng-sessiond/syscall.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/src/bin/lttng-sessiond/syscall.c b/src/bin/lttng-sessiond/syscall.c
index 6ee38bd..7d0a92b 100644
--- a/src/bin/lttng-sessiond/syscall.c
+++ b/src/bin/lttng-sessiond/syscall.c
@@ -16,6 +16,8 @@
  */
 
 #define _LGPL_SOURCE
+#include <stdbool.h>
+
 #include <common/bitfield.h>
 #include <common/common.h>
 #include <common/kernel-ctl/kernel-ctl.h>
@@ -43,7 +45,8 @@ int syscall_init_table(void)
 	size_t nbmem;
 	FILE *fp;
 	/* Syscall data from the kernel. */
-	size_t index;
+	size_t index = 0;
+	bool at_least_one_syscall = false;
 	uint32_t bitness;
 	char name[SYSCALL_NAME_LEN];
 
@@ -76,12 +79,13 @@ int syscall_init_table(void)
 				name = %" XSTR(SYSCALL_NAME_LEN) "[^;]; \
 				bitness = %u; };\n",
 				&index, name, &bitness) == 3) {
-		if (index >= nbmem ) {
+		at_least_one_syscall = true;
+		if (index >= nbmem) {
 			struct syscall *new_list;
 			size_t new_nbmem;
 
 			/* Double memory size. */
-			new_nbmem = max(index, nbmem << 1);
+			new_nbmem = max(index + 1, nbmem << 1);
 			if (new_nbmem > (SIZE_MAX / sizeof(*new_list))) {
 				/* Overflow, stop everything, something went really wrong. */
 				ERR("Syscall listing memory size overflow. Stopping");
@@ -123,7 +127,10 @@ int syscall_init_table(void)
 		*/
 	}
 
-	syscall_table_nb_entry = index;
+	/* Index starts at 0. */
+	if (at_least_one_syscall) {
+		syscall_table_nb_entry = index + 1;
+	}
 
 	ret = 0;
 
-- 
2.7.4

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