From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta1.formilux.org (mta1.formilux.org [51.159.59.229]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4C38C33B94A for ; Sat, 7 Feb 2026 19:12:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=51.159.59.229 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770491553; cv=none; b=Shn7uFfR5E2JHYMrB98qknvpwnlU8Le+nZ14hZuuXpgPJoSfjHKUKDjX0AdOZjR1cgh3XPZjAM3nzINK1gyCYFjrwpBOWmA0SbDFZ4XKTirtFSSBqLh1HNkN3c0D4OoNmjOO7rI1N5rqqLboYCOlAlllmQa0harytPKdHoGH5vI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770491553; c=relaxed/simple; bh=W17SUzawu2ck2+PAb3WWTBY976rlLEmV7S1Uk4xOHYs=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Kx3nUKdqfS1ffrKpIZxqc2eLiGULY5ogNH0WOd+YbIEeBg8mfGuWQ/xss6PfWZdYTw9NeRV2RFSNrTF7u2Rpy/KMSIa//t5ldNAJ3DrOX/0h1p4X0lQLtl1vNzUTu+AhoEL7urY5/hQUxHulFRrHTR6Rxn3qRqMb9A8BfY1rjdc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=1wt.eu; spf=pass smtp.mailfrom=1wt.eu; dkim=pass (1024-bit key) header.d=1wt.eu header.i=@1wt.eu header.b=isISGHUE; arc=none smtp.client-ip=51.159.59.229 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=1wt.eu Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=1wt.eu Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=1wt.eu header.i=@1wt.eu header.b="isISGHUE" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1wt.eu; s=mail; t=1770491550; bh=zATy60SKlwn8Jw+R0iDUOYz5JUn1m3bF4PKjT/fYooE=; h=From:Message-ID:From; b=isISGHUEoqHICMPdlWk/HBBPeA648tLLMPMxQSfTQqCASqos0vt1XFvpP+Oj5eSEO VS9wA/gohv2uqifdn8xMfkGNEDALRNlzeb1wVRc6xvGCpCeFuvcxBBgUSRRbA814m+ EN0UyxhEHOpPM6rG4+TU/pihGB4fNUPLcRsRghe0= Received: from 1wt.eu (ded1.1wt.eu [163.172.96.212]) by mta1.formilux.org (Postfix) with ESMTP id 9D471C0B86; Sat, 07 Feb 2026 20:12:30 +0100 (CET) Date: Sat, 7 Feb 2026 20:12:30 +0100 From: Willy Tarreau To: david.laight.linux@gmail.com Cc: Thomas =?iso-8859-1?Q?Wei=DFschuh?= , linux-kernel@vger.kernel.org, Cheng Li Subject: Re: [PATCH v2 next 02/11] tools/nolibc/printf: Move snprintf length check to callback Message-ID: References: <20260206191121.3602-1-david.laight.linux@gmail.com> <20260206191121.3602-3-david.laight.linux@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260206191121.3602-3-david.laight.linux@gmail.com> On Fri, Feb 06, 2026 at 07:11:12PM +0000, david.laight.linux@gmail.com wrote: > From: David Laight > > Move output truncation to the snprintf() callback. > This simplifies the main code and ensures the truncation will be > correct when left-alignment is added. > > Add a zero length callback to 'finalise' the buffer rather than > doing it in snprintf() itself. > > Signed-off-by: David Laight > --- > > Changes for v2: > - Formally patch 1 > - Add comments about the final callback. > > tools/include/nolibc/stdio.h | 68 ++++++++++++++++++++++-------------- > 1 file changed, 41 insertions(+), 27 deletions(-) > > diff --git a/tools/include/nolibc/stdio.h b/tools/include/nolibc/stdio.h > index f162cc697a73..36733ecd4261 100644 > --- a/tools/include/nolibc/stdio.h > +++ b/tools/include/nolibc/stdio.h > @@ -245,15 +245,15 @@ char *fgets(char *s, int size, FILE *stream) > * - %s > * - unknown modifiers are ignored. > */ > -typedef int (*__nolibc_printf_cb)(intptr_t state, const char *buf, size_t size); > +typedef int (*__nolibc_printf_cb)(void *state, const char *buf, size_t size); > > -static __attribute__((unused, format(printf, 4, 0))) > -int __nolibc_printf(__nolibc_printf_cb cb, intptr_t state, size_t n, const char *fmt, va_list args) > +static __attribute__((unused, format(printf, 3, 0))) > +int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list args) > { > char escape, lpref, ch; > unsigned long long v; > unsigned int written, width; > - size_t len, ofs, w; > + size_t len, ofs; > char tmpbuf[21]; > const char *outstr; > > @@ -355,17 +355,13 @@ int __nolibc_printf(__nolibc_printf_cb cb, intptr_t state, size_t n, const char > outstr = fmt; > len = ofs - 1; > flush_str: > - if (n) { > - w = len < n ? len : n; > - n -= w; > - while (width-- > w) { > - if (cb(state, " ", 1) != 0) > - return -1; > - written += 1; > - } > - if (cb(state, outstr, w) != 0) > + while (width-- > len) { > + if (cb(state, " ", 1) != 0) > return -1; > + written += 1; > } > + if (cb(state, outstr, len) != 0) > + return -1; > > written += len; > do_escape: > @@ -378,18 +374,23 @@ int __nolibc_printf(__nolibc_printf_cb cb, intptr_t state, size_t n, const char > > /* literal char, just queue it */ > } > + > + /* Flush/terminate any buffer. */ > + if (cb(state, NULL, 0) != 0) > + return -1; > + I suspect this hunk is in fact part of the next patch which adds buffering, because I don't see what there is to flush here without any buffer. If really needed, then I think that it's about time to start adding a comment about the __nolibc_printf() function to explain how it's supposed to work, because callback-driven code is unreadable, there are hidden expectations everywhere that are super hard to guess or verify. > return written; > } > > -static int __nolibc_fprintf_cb(intptr_t state, const char *buf, size_t size) > +static int __nolibc_fprintf_cb(void *stream, const char *buf, size_t size) I must confess I'm not a big fan of the void* here. I've seen that you're having one state for snprintf() and another one for fprintf(), maybe they could be efficiently merged into a common printf_state ? Note that I'm not vetoing this, I just want to be convinced that it's the best choice, and neither the code, comments nor commit messages for now suggest so. > { > - return _fwrite(buf, size, (FILE *)state); > + return size ? _fwrite(buf, size, stream) : 0; > } > > static __attribute__((unused, format(printf, 2, 0))) > int vfprintf(FILE *stream, const char *fmt, va_list args) > { > - return __nolibc_printf(__nolibc_fprintf_cb, (intptr_t)stream, SIZE_MAX, fmt, args); > + return __nolibc_printf(__nolibc_fprintf_cb, stream, fmt, args); > } > > static __attribute__((unused, format(printf, 1, 0))) > @@ -447,26 +448,39 @@ int dprintf(int fd, const char *fmt, ...) > return ret; > } > > -static int __nolibc_sprintf_cb(intptr_t _state, const char *buf, size_t size) > +struct __nolibc_sprintf_cb_state { > + char *buf; > + size_t size; > +}; > + > +static int __nolibc_sprintf_cb(void *v_state, const char *buf, size_t size) > { > - char **state = (char **)_state; > + struct __nolibc_sprintf_cb_state *state = v_state; > + char *tgt; > > - memcpy(*state, buf, size); > - *state += size; > + if (size >= state->size) { > + if (state->size <= 1) > + return 0; I failed to understand that one. Don't we want to at least write the trailing zero when there's one byte left ? A short comment explaining that case would help. > + size = state->size - 1; > + } > + tgt = state->buf; > + if (size) { > + state->size -= size; > + state->buf = tgt + size; > + memcpy(tgt, buf, size); > + } else { > + /* In particular from cb(NULL, 0) at the end of __nolibc_printf(). */ > + *tgt = '\0'; > + } Usually, "if/else" constructs result in larger code due to jumps. Here we certainly can unconditionally write the trailing zero. Bingo, we're saving 9 bytes on x86_64 by moving it above. And even 17 bytes by dropping the test on size and updating the state after the memcpy: if (size >= state->size) { if (state->size <= 1) return 0; size = state->size - 1; } *state->buf = '\0'; memcpy(state->buf, buf, size); state->buf += size; state->size -= size; snprintf-patch1:000000000000003e t __nolibc_sprintf_cb snprintf-patch1-alt1:0000000000000033 t __nolibc_sprintf_cb snprintf-patch1-alt2:000000000000002d t __nolibc_sprintf_cb (not tested but worth a try). > return 0; > } > > static __attribute__((unused, format(printf, 3, 0))) > int vsnprintf(char *buf, size_t size, const char *fmt, va_list args) > { > - char *state = buf; > - int ret; > + struct __nolibc_sprintf_cb_state state = { .buf = buf, .size = size }; > > - ret = __nolibc_printf(__nolibc_sprintf_cb, (intptr_t)&state, size, fmt, args); > - if (ret < 0) > - return ret; > - buf[(size_t)ret < size ? (size_t)ret : size - 1] = '\0'; > - return ret; > + return __nolibc_printf(__nolibc_sprintf_cb, &state, fmt, args); > } > > static __attribute__((unused, format(printf, 3, 4))) Willy