At Tue, 12 Oct 2010 10:35:07 +0100, Mark Brown wrote:
On Mon, Oct 11, 2010 at 10:57:40PM +0200, Takashi Iwai wrote:
Dan Carpenter wrote:
Well, actually we should fix either:
check the return of snprintf() at each time properly,
list_for_each_entry(dai, &dai_list, list) { int len = snprintf(buf + ret, PAGE_SIZE - ret, "%s\n", dai->name); if (len < 0) return len; ret += len; }
In this case we're deliberately eating the error since all these files are about getting diagnostics out - the code is intentionally soldiering on and trying to get as much data out as possible rather than giving up on error.
Then replace return with continue. But, the error doesn't occur in the case of kernel snprintf(), so it should be no issue.
In either case, a negative check after for loop is superfluous.
In those ones, yes - it's pretty much there for paranoia since the copy to userspace is more likely to explode than random memory corruption.
Well, the point is that checking the error at that point is logically wrong and pointless. It's not about optimization or such. It's about the code logic.
And, when no negative return value is assured (or filtered out like above), there can't be overflow, too. snprintf() fills the string at most the size including NUL-char. OTOH, it returns the size that doesn't include NUL-char.
Dan was saying that it would return sizes larger than the string it wrote (which is a behaviour of some implementations) which would be an issue since it would cause us to pass bad buffer pointers into subsequent snprintf() calls.
I've not had time to look at this properly but Dan's analysis seems off.
Argh, yes, I'm (again) confused by that behavior. The problem is the potential buffer overflow, indeed. snprintf() returns the size that would be printed. Thus a safe code would be like:
list_for_each_entry(dai, &dai_list, list) { int len = snprintf(buf + ret, PAGE_SIZE - ret, "%s\n", dai->name); if (len < 0) continue; ret += len; if (ret >= PAGE_SIZE) { ret = PAGE_SIZE; break; } }
Or, by assumption of non-negative return,
list_for_each_entry(dai, &dai_list, list) { ret += snprintf(buf + ret, PAGE_SIZE - ret, "%s\n", dai->name); if (ret >= PAGE_SIZE) { ret = PAGE_SIZE; break; } }
Takashi