From aec0bba106d8bce829671ca8659ad338aa677e9f Mon Sep 17 00:00:00 2001 From: Jeff King Date: Tue, 4 Aug 2020 03:43:53 -0400 Subject: [PATCH 1/3] config: work around gcc-10 -Wstringop-overflow warning MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Compiling with gcc-10, -O2, and -fsanitize=undefined results in a compiler warning: config.c: In function ‘git_config_copy_or_rename_section_in_file’: config.c:3170:17: error: writing 1 byte into a region of size 0 [-Werror=stringop-overflow=] 3170 | output[0] = '\t'; | ~~~~~~~~~~^~~~~~ config.c:3076:7: note: at offset -1 to object ‘buf’ with size 1024 declared here 3076 | char buf[1024]; | ^~~ This is a false positive. The interesting lines of code are: int i; char *output = buf; ... for (i = 0; buf[i] && isspace(buf[i]); i++) ; /* do nothing */ ... int offset; offset = section_name_match(&buf[i], old_name); if (offset > 0) { ... output += offset + i; if (strlen(output) > 0) { /* * More content means there's * a declaration to put on the * next line; indent with a * tab */ output -= 1; output[0] = '\t'; } } So we do assign output to buf initially. Later we increment it based on "offset" and "i" and then subtract "1" from it. That latter step is what the compiler is complaining about; it could lead to going off the left side of the array if "output == buf" at the moment of the subtraction. For that to be the case, then "offset + i" would have to be 0. But that can't happen: - we know that "offset" is at least 1, since we're in a conditional block that checks that - we know that "i" is not negative, since it started at 0 and only incremented over whitespace So the sum must be at least 1, and therefore it's OK to subtract one from "output". But that's not quite the whole story. Since "i" is an int, it could in theory be possible to overflow to negative (when counting whitespace on a very large string). But we know that's impossible because we're counting the 1024-byte buffer we just fed to fgets(), so it can never be larger than that. Switching the type of "i" to "unsigned" makes the warning go away, so let's do that. Arguably size_t is an even better type (for this and for the other length fields), but switching to it produces a similar but distinct warning: config.c: In function ‘git_config_copy_or_rename_section_in_file’: config.c:3170:13: error: array subscript -1 is outside array bounds of ‘char[1024]’ [-Werror=array-bounds] 3170 | output[0] = '\t'; | ~~~~~~^~~ config.c:3076:7: note: while referencing ‘buf’ 3076 | char buf[1024]; | ^~~ If we were to ever switch off of fgets() to strbuf_getline() or similar, we'd probably need to use size_t to avoid other overflow problems. But for now we know we're safe because of the small fixed size of our buffer. Signed-off-by: Jeff King Signed-off-by: Junio C Hamano --- config.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/config.c b/config.c index 8db9c77098..2b79fe76ad 100644 --- a/config.c +++ b/config.c @@ -3115,7 +3115,7 @@ static int git_config_copy_or_rename_section_in_file(const char *config_filename } while (fgets(buf, sizeof(buf), config_file)) { - int i; + unsigned i; int length; int is_section = 0; char *output = buf; From fd9a631c56ff326bea2956b675f205cd474def4e Mon Sep 17 00:00:00 2001 From: Jeff King Date: Tue, 4 Aug 2020 03:46:52 -0400 Subject: [PATCH 2/3] revision: avoid out-of-bounds read/write on empty pathspec Running t4216 with ASan results in it complaining of an out-of-bounds read in prepare_to_use_bloom_filter(). The issue is this code to strip a trailing slash: last_index = pi->len - 1; if (pi->match[last_index] == '/') { because we have no guarantee that pi->len isn't zero. This can happen if the pathspec is ".", as we translate that to an empty string. And if that read of random memory does trigger the conditional, we'd then do an out-of-bounds write: path_alloc = xstrdup(pi->match); path_alloc[last_index] = '\0'; Let's make sure to check the length before subtracting. Note that for an empty pathspec, we'd end up bailing from the function a few lines later, which makes it tempting to just: if (!pi->len) return; early here. But our code here is stripping a trailing slash, and we need to check for emptiness after stripping that slash, too. So we'd have two blocks, which would require repeating some cleanup code. Instead, just skip the trailing-slash for an empty string. Setting last_index at all in the case is awkward since it will have a nonsense value (and it uses an "int", which is a too-small type for a string anyway). So while we're here, let's: - drop last_index entirely; it's only used in two spots right next to each other and writing out "pi->len - 1" in both is actually easier to follow - use xmemdupz() to duplicate the string. This is slightly more efficient, but more importantly makes the intent more clear by allocating the correct-sized substring in the first place. It also eliminates any question of whether path_alloc is as long as pi->match (which it would not be if pi->match has any embedded NULs, though in practice this is probably impossible). Signed-off-by: Jeff King Signed-off-by: Junio C Hamano --- revision.c | 7 ++----- 1 file changed, 2 insertions(+), 5 deletions(-) diff --git a/revision.c b/revision.c index 6de29cdf7a..5ed86e4524 100644 --- a/revision.c +++ b/revision.c @@ -669,7 +669,6 @@ static void prepare_to_use_bloom_filter(struct rev_info *revs) struct pathspec_item *pi; char *path_alloc = NULL; const char *path, *p; - int last_index; size_t len; int path_component_nr = 1; @@ -692,12 +691,10 @@ static void prepare_to_use_bloom_filter(struct rev_info *revs) return; pi = &revs->pruning.pathspec.items[0]; - last_index = pi->len - 1; /* remove single trailing slash from path, if needed */ - if (pi->match[last_index] == '/') { - path_alloc = xstrdup(pi->match); - path_alloc[last_index] = '\0'; + if (pi->len > 0 && pi->match[pi->len - 1] == '/') { + path_alloc = xmemdupz(pi->match, pi->len - 1); path = path_alloc; } else path = pi->match; From 398e659e1ec60501d67a0f3cb1a1052c6e50038c Mon Sep 17 00:00:00 2001 From: Jeff King Date: Tue, 4 Aug 2020 03:50:17 -0400 Subject: [PATCH 3/3] revision: avoid leak when preparing bloom filter for "/" If we're given an empty pathspec, we refuse to set up bloom filters, as described in f3c2a36810 (revision: empty pathspecs should not use Bloom filters, 2020-07-01). But before the empty string check, we drop any trailing slash by allocating a new string without it. So a pathspec consisting only of "/" will allocate that string, but then still cause us to bail, leaking the new string. Let's make sure to free it. Signed-off-by: Jeff King Signed-off-by: Junio C Hamano --- revision.c | 1 + 1 file changed, 1 insertion(+) diff --git a/revision.c b/revision.c index 5ed86e4524..b80868556b 100644 --- a/revision.c +++ b/revision.c @@ -702,6 +702,7 @@ static void prepare_to_use_bloom_filter(struct rev_info *revs) len = strlen(path); if (!len) { revs->bloom_filter_settings = NULL; + free(path_alloc); return; }