GCC overflow warnings (format-overflow, stringop-overflow)

[tiran]

BPO	40952
Nosy	@ncoghlan, @tiran

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-06-12.07:14:45.817>
labels = ['build', '3.9', '3.10']
title = 'GCC overflow warnings (format-overflow, stringop-overflow)'
updated_at = <Date 2020-11-07.05:56:33.375>
user = 'https://github.com/tiran'

bugs.python.org fields:

activity = <Date 2020-11-07.05:56:33.375>
actor = 'ncoghlan'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Build']
creation = <Date 2020-06-12.07:14:45.817>
creator = 'christian.heimes'
dependencies = []
files = []
hgrepos = []
issue_num = 40952
keywords = []
message_count = 2.0
messages = ['371335', '380493']
nosy_count = 2.0
nosy_names = ['ncoghlan', 'christian.heimes']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'compile error'
url = 'https://bugs.python.org/issue40952'
versions = ['Python 3.9', 'Python 3.10']

[tiran]

I'm getting a couple of compiler warnings with gcc-10.1.1 (Fedora 32) with an asan and ubsan build:

Parser/string_parser.c: In function ‘decode_unicode_with_escapes’:
Parser/string_parser.c:100:17: warning: null destination pointer [-Wformat-overflow=]
100 | sprintf(p, "\\U%08x", chr);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~
Parser/string_parser.c:100:17: warning: null destination pointer [-Wformat-overflow=]
Parser/string_parser.c:100:17: warning: null destination pointer [-Wformat-overflow=]

Objects/unicodeobject.c: In function ‘xmlcharrefreplace’:
Objects/unicodeobject.c:849:16: warning: null destination pointer [-Wformat-overflow=]
849 | str += sprintf(str, "&#%d;", PyUnicode_READ(kind, data, i));
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Objects/unicodeobject.c:849:16: warning: null destination pointer [-Wformat-overflow=]
Objects/unicodeobject.c:849:16: warning: null destination pointer [-Wformat-overflow=]

In function ‘assemble_lnotab’,
inlined from ‘assemble_emit’ at Python/compile.c:5697:25,
inlined from ‘assemble’ at Python/compile.c:6036:18:
Python/compile.c:5651:19: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
5651 | *lnotab++ = k;
| ~~~~~~~~~~^~~

[ncoghlan]

I *think* the lnotab one is the compiler failing to detect that the pointer has been updated to point inside the body of a Python object, but I'm also not 100% sure that it's a false alarm.

[skirpichev]

Another case for stringop-overflow on GCC 12.2.0 (Debian Bookworm): #108562 (EXTRACT_DIGIT macro expansion). Haven't seen them on GCC 10.2.x (Debian oldstable).