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Thus, >>> the return value of 'len_left' means that the last character has been >>> dropped. >> >> should we have test for this in progs/test_snprintf.c ? > > It might be too annoying to set up such test, and given the fix is > pretty trivial IMO it's ok without extra test. But cc Alan for ack. > Alan, please take a look as well. > I can follow up with a test, it should be okay I think (we can use the "don't show types" flag and tryp to print "10" with a 2-byte len or similar). In terms of the fix, it looks good, but given that the code is tricky, it might be good to expand a bit on the explanation. Something like the below? "When using btf_type_snprintf_show(), the user passes in a "len" value, and we use it to initialize ssnprintf.len_left, indicating how much space remains for the string representation, including the null byte, so "len - 1" bytes are actually available for the actual string data, leaving one for the terminating null byte. In btf_snprintf_show() - which is passed the ssnprintf data as an argument - vsnprintf() returns the len that would have been written, and this _excludes_ the null terminator. But we want to handle cases where the length of the string to be written (excluding the null terminator) exactly matches the original len value we passed in (len == len_left) in the same way was we do other overflow cases (len > len_left)." Acked-by: Alan Maguire >> >> jirka >> >>> >>> Signed-off-by: Fedor Tokarev >>> --- >>> kernel/bpf/btf.c | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c >>> index eb12d4f705cc..a9c1c98017d4 100644 >>> --- a/kernel/bpf/btf.c >>> +++ b/kernel/bpf/btf.c >>> @@ -6519,7 +6519,7 @@ static void btf_snprintf_show(struct btf_show *show, const char *fmt, >>> if (len < 0) { >>> ssnprintf->len_left = 0; >>> ssnprintf->len = len; >>> - } else if (len > ssnprintf->len_left) { >>> + } else if (len >= ssnprintf->len_left) { >>> /* no space, drive on to get length we would have written */ >>> ssnprintf->len_left = 0; >>> ssnprintf->len += len; >>> -- >>> 2.25.1 >>>