119 lines
5.2 KiB
Diff
119 lines
5.2 KiB
Diff
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From 3285ced5cbec5d8a0c3464f1762d34439280f8eb Mon Sep 17 00:00:00 2001
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From: Willy Tarreau <w@1wt.eu>
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Date: Wed, 11 Mar 2020 11:54:04 +0100
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Subject: [PATCH] BUG/MAJOR: list: fix invalid element address calculation
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Ryan O'Hara reported that haproxy breaks on fedora-32 using gcc-10
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(pre-release). It turns out that constructs such as:
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while (item != head) {
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item = LIST_ELEM(item.n);
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}
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loop forever, never matching <item> to <head> despite a printf there
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showing them equal. In practice the problem is that the LIST_ELEM()
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macro is wrong, it assigns the subtract of two pointers (an integer)
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to another pointer through a cast to its pointer type. And GCC 10 now
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considers that this cannot match a pointer and silently optimizes the
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comparison away. A tested workaround for this is to build with
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-fno-tree-pta. Note that older gcc versions even with -ftree-pta do
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not exhibit this rather surprizing behavior.
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This patch changes the test to instead cast the null-based address to
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an int to get the offset and subtract it from the pointer, and this
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time it works. There were just a few places to adjust. Ideally
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offsetof() should be used but the LIST_ELEM() API doesn't make this
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trivial as it's commonly called with a typeof(ptr) and not typeof(ptr*)
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thus it would require to completely change the whole API, which is not
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something workable in the short term, especially for a backport.
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With this change, the emitted code is subtly different even on older
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versions. A code size reduction of ~600 bytes and a total executable
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size reduction of ~1kB are expected to be observed and should not be
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taken as an anomaly. Typically this loop in dequeue_proxy_listeners() :
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while ((listener = MT_LIST_POP(...)))
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used to produce this code where the comparison is performed on RAX
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while the new offset is assigned to RDI even though both are always
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identical:
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53ded8: 48 8d 78 c0 lea -0x40(%rax),%rdi
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53dedc: 48 83 f8 40 cmp $0x40,%rax
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53dee0: 74 39 je 53df1b <dequeue_proxy_listeners+0xab>
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and now produces this one which is slightly more efficient as the
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same register is used for both purposes:
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53dd08: 48 83 ef 40 sub $0x40,%rdi
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53dd0c: 74 2d je 53dd3b <dequeue_proxy_listeners+0x9b>
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Similarly, retrieving the channel from a stream_interface using si_ic()
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and si_oc() used to cause this (stream-int in rdi):
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1cb7: c7 47 1c 00 02 00 00 movl $0x200,0x1c(%rdi)
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1cbe: f6 47 04 10 testb $0x10,0x4(%rdi)
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1cc2: 74 1c je 1ce0 <si_report_error+0x30>
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1cc4: 48 81 ef 00 03 00 00 sub $0x300,%rdi
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1ccb: 81 4f 10 00 08 00 00 orl $0x800,0x10(%rdi)
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and now causes this:
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1cb7: c7 47 1c 00 02 00 00 movl $0x200,0x1c(%rdi)
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1cbe: f6 47 04 10 testb $0x10,0x4(%rdi)
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1cc2: 74 1c je 1ce0 <si_report_error+0x30>
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1cc4: 81 8f 10 fd ff ff 00 orl $0x800,-0x2f0(%rdi)
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There is extremely little chance that this fix wakes up a dormant bug as
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the emitted code effectively does what the source code intends.
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This must be backported to all supported branches (dropping MT_LIST_ELEM
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and the spoa_example parts as needed), since the bug is subtle and may
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not always be visible even when compiling with gcc-10.
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(cherry picked from commit 855796bdc8b1bc2ebcc697ca99fb304bc6e2abe9)
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Signed-off-by: Willy Tarreau <w@1wt.eu>
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---
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contrib/spoa_example/include/mini-clist.h | 2 +-
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include/common/mini-clist.h | 4 ++--
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2 files changed, 3 insertions(+), 3 deletions(-)
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diff --git a/contrib/spoa_example/include/mini-clist.h b/contrib/spoa_example/include/mini-clist.h
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index a89255c46..d009704cc 100644
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--- a/contrib/spoa_example/include/mini-clist.h
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+++ b/contrib/spoa_example/include/mini-clist.h
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@@ -44,7 +44,7 @@ struct list {
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* since it's used only once.
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* Example: LIST_ELEM(cur_node->args.next, struct node *, args)
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*/
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-#define LIST_ELEM(lh, pt, el) ((pt)(((void *)(lh)) - ((void *)&((pt)NULL)->el)))
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+#define LIST_ELEM(lh, pt, el) ((pt)(((const char *)(lh)) - ((size_t)&((pt)NULL)->el)))
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/* checks if the list head <lh> is empty or not */
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#define LIST_ISEMPTY(lh) ((lh)->n == (lh))
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diff --git a/include/common/mini-clist.h b/include/common/mini-clist.h
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index e29451148..01bd0b695 100644
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--- a/include/common/mini-clist.h
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+++ b/include/common/mini-clist.h
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@@ -137,7 +137,7 @@ struct cond_wordlist {
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* since it's used only once.
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* Example: LIST_ELEM(cur_node->args.next, struct node *, args)
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*/
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-#define LIST_ELEM(lh, pt, el) ((pt)(((void *)(lh)) - ((void *)&((pt)NULL)->el)))
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+#define LIST_ELEM(lh, pt, el) ((pt)(((const char *)(lh)) - ((size_t)&((pt)NULL)->el)))
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/* checks if the list head <lh> is empty or not */
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#define LIST_ISEMPTY(lh) ((lh)->n == (lh))
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@@ -463,7 +463,7 @@ struct cond_wordlist {
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* since it's used only once.
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* Example: MT_LIST_ELEM(cur_node->args.next, struct node *, args)
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*/
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-#define MT_LIST_ELEM(lh, pt, el) ((pt)(((void *)(lh)) - ((void *)&((pt)NULL)->el)))
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+#define MT_LIST_ELEM(lh, pt, el) ((pt)(((const char *)(lh)) - ((size_t)&((pt)NULL)->el)))
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/* checks if the list head <lh> is empty or not */
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#define MT_LIST_ISEMPTY(lh) ((lh)->next == (lh))
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--
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2.24.1
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