From: Joe Simmons-Talbott <josimmon@redhat.com>
To: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>,
libc-alpha@sourceware.org
Subject: Re: [PATCH] resolv/res_query: Add note indicating that alloca usage is safe.
Date: Thu, 14 Sep 2023 11:04:25 -0400 [thread overview]
Message-ID: <20230914150425.GQ3849957@oak> (raw)
In-Reply-To: <20230828145825.GX3849957@oak>
Ping.
On Mon, Aug 28, 2023 at 10:58:25AM -0400, Joe Simmons-Talbott via Libc-alpha wrote:
> On Mon, Aug 28, 2023 at 10:50:21AM -0300, Adhemerval Zanella Netto wrote:
> >
> >
> > On 05/07/23 15:13, Joe Simmons-Talbott via Libc-alpha wrote:
> > > The buffer size is small (< 1024) and fixed sized so alloca is safe
> > > here.
> > > ---
> > > resolv/res_query.c | 1 +
> > > 1 file changed, 1 insertion(+)
> > >
> > > diff --git a/resolv/res_query.c b/resolv/res_query.c
> > > index 049de91b95..0e0e7be624 100644
> > > --- a/resolv/res_query.c
> > > +++ b/resolv/res_query.c
> > > @@ -117,6 +117,7 @@ __res_context_query (struct resolv_context *ctx, const char *name,
> > > int n, use_malloc = 0;
> > >
> > > size_t bufsize = (type == T_QUERY_A_AND_AAAA ? 2 : 1) * QUERYSIZE;
> > > + /* alloca is safe here since bufsize < 1024 and fixed sized. */
> > > u_char *buf = alloca (bufsize);
> >
> > The bufsize on current Linux build is:
> >
> > size_t bufsize = (type == 439963904 ? 2 : 1) * (12 + 4 + 255 + 1);
> >
> > So with upper bound as 544 (2 * (12 + 4 + 255 + 1)). However, it might
> > increase to 2 * PACKETSIZE later with malloc. This is exactly the scenarion
> > scratch_buffer was created, so maybe we should use it. Below a complete
> > untested patch:
>
> Thanks for the patch. I've tested it with 'make check' on
> x86_64-linux-gnu. I'm not sure how to handle this and don't feel
> comfortable submitting it on your behalf or as my own work but am happy
> to give a Reviewed-by once there's a commit message.
>
> Thanks,
> Joe
> >
> > diff --git a/resolv/res_query.c b/resolv/res_query.c
> > index 049de91b95..b234db83c1 100644
> > --- a/resolv/res_query.c
> > +++ b/resolv/res_query.c
> > @@ -80,6 +80,7 @@
> > #include <stdlib.h>
> > #include <string.h>
> > #include <shlib-compat.h>
> > +#include <scratch_buffer.h>
> >
> > #if PACKETSZ > 65536
> > #define MAXPACKET PACKETSZ
> > @@ -114,11 +115,14 @@ __res_context_query (struct resolv_context *ctx, const char *name,
> > struct __res_state *statp = ctx->resp;
> > UHEADER *hp = (UHEADER *) answer;
> > UHEADER *hp2;
> > - int n, use_malloc = 0;
> > -
> > - size_t bufsize = (type == T_QUERY_A_AND_AAAA ? 2 : 1) * QUERYSIZE;
> > - u_char *buf = alloca (bufsize);
> > - u_char *query1 = buf;
> > + int n;
> > +
> > + /* It requires 2 times QUERYSIZE for type == T_QUERY_A_AND_AAAA. */
> > + struct scratch_buffer buf;
> > + scratch_buffer_init (&buf);
> > + _Static_assert (2 * QUERYSIZE <= sizeof (buf.__space.__c),
> > + "scratch_buffer too small");
> > + u_char *query1 = buf.data;
> > int nquery1 = -1;
> > u_char *query2 = NULL;
> > int nquery2 = 0;
> > @@ -129,14 +133,14 @@ __res_context_query (struct resolv_context *ctx, const char *name,
> > if (type == T_QUERY_A_AND_AAAA)
> > {
> > n = __res_context_mkquery (ctx, QUERY, name, class, T_A, NULL,
> > - query1, bufsize);
> > + query1, buf.length);
> > if (n > 0)
> > {
> > if ((statp->options & (RES_USE_EDNS0|RES_USE_DNSSEC)) != 0)
> > {
> > /* Use RESOLV_EDNS_BUFFER_SIZE because the receive
> > buffer can be reallocated. */
> > - n = __res_nopt (ctx, n, query1, bufsize,
> > + n = __res_nopt (ctx, n, query1, buf.length,
> > RESOLV_EDNS_BUFFER_SIZE);
> > if (n < 0)
> > goto unspec_nomem;
> > @@ -146,20 +150,20 @@ __res_context_query (struct resolv_context *ctx, const char *name,
> > /* Align the buffer. */
> > int npad = ((nquery1 + __alignof__ (HEADER) - 1)
> > & ~(__alignof__ (HEADER) - 1)) - nquery1;
> > - if (n > bufsize - npad)
> > + if (n > buf.length - npad)
> > {
> > n = -1;
> > goto unspec_nomem;
> > }
> > int nused = n + npad;
> > - query2 = buf + nused;
> > + query2 = buf.data + nused;
> > n = __res_context_mkquery (ctx, QUERY, name, class, T_AAAA,
> > - NULL, query2, bufsize - nused);
> > + NULL, query2, buf.length - nused);
> > if (n > 0
> > && (statp->options & (RES_USE_EDNS0|RES_USE_DNSSEC)) != 0)
> > /* Use RESOLV_EDNS_BUFFER_SIZE because the receive
> > buffer can be reallocated. */
> > - n = __res_nopt (ctx, n, query2, bufsize,
> > + n = __res_nopt (ctx, n, query2, buf.length,
> > RESOLV_EDNS_BUFFER_SIZE);
> > nquery2 = n;
> > }
> > @@ -169,7 +173,7 @@ __res_context_query (struct resolv_context *ctx, const char *name,
> > else
> > {
> > n = __res_context_mkquery (ctx, QUERY, name, class, type, NULL,
> > - query1, bufsize);
> > + query1, buf.length);
> >
> > if (n > 0
> > && (statp->options & (RES_USE_EDNS0|RES_USE_DNSSEC)) != 0)
> > @@ -181,27 +185,25 @@ __res_context_query (struct resolv_context *ctx, const char *name,
> > advertise = anslen;
> > else
> > advertise = RESOLV_EDNS_BUFFER_SIZE;
> > - n = __res_nopt (ctx, n, query1, bufsize, advertise);
> > + n = __res_nopt (ctx, n, query1, buf.length, advertise);
> > }
> >
> > nquery1 = n;
> > }
> >
> > - if (__glibc_unlikely (n <= 0) && !use_malloc) {
> > + if (__glibc_unlikely (n <= 0)) {
> > /* Retry just in case res_nmkquery failed because of too
> > short buffer. Shouldn't happen. */
> > - bufsize = (type == T_QUERY_A_AND_AAAA ? 2 : 1) * MAXPACKET;
> > - buf = malloc (bufsize);
> > - if (buf != NULL) {
> > - query1 = buf;
> > - use_malloc = 1;
> > + if (scratch_buffer_set_array_size (&buf,
> > + T_QUERY_A_AND_AAAA ? 2 : 1,
> > + MAXPACKET)) {
> > + query1 = buf.data;
> > goto again;
> > }
> > }
> > if (__glibc_unlikely (n <= 0)) {
> > RES_SET_H_ERRNO(statp, NO_RECOVERY);
> > - if (use_malloc)
> > - free (buf);
> > + scratch_buffer_free (&buf);
> > return (n);
> > }
> >
> > @@ -224,8 +226,7 @@ __res_context_query (struct resolv_context *ctx, const char *name,
> > answerp2_malloced);
> > }
> >
> > - if (use_malloc)
> > - free (buf);
> > + scratch_buffer_free (&buf);
> > if (n < 0) {
> > RES_SET_H_ERRNO(statp, TRY_AGAIN);
> > return (n);
> >
>
next prev parent reply other threads:[~2023-09-14 15:04 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-07-05 18:13 Joe Simmons-Talbott
2023-08-10 13:46 ` Joe Simmons-Talbott
2023-08-28 13:21 ` Joe Simmons-Talbott
2023-08-28 13:50 ` Adhemerval Zanella Netto
2023-08-28 14:58 ` Joe Simmons-Talbott
2023-09-14 15:04 ` Joe Simmons-Talbott [this message]
2023-09-14 17:36 ` Adhemerval Zanella Netto
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