collnet: size shared collNet comm arrays by NCCL_MAX_NETDEVS - #2262
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EylonKrause wants to merge 1 commit into
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collnet: size shared collNet comm arrays by NCCL_MAX_NETDEVS#2262EylonKrause wants to merge 1 commit into
EylonKrause wants to merge 1 commit into
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struct sharedResources sized collNetListenComms / collNetComms by MAXCHANNELS (64) but indexes them by netDev, which ranges up to NCCL_MAX_NETDEVS (128) -- the same domain used by the adjacent commRefCount, the sibling proxyAppend[2*NCCL_MAX_NETDEVS], and regular-net netComms[NCCL_MAX_NETDEVS]. On a host where the selected collnet netDev index is >= 64 this reads/writes past the arrays into neighbouring fields. Size both arrays by NCCL_MAX_NETDEVS to match their index domain. Signed-off-by: EylonKrause <eylon1909@gmail.com>
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Disclosure: this contribution was authored with an AI coding assistant (Claude) and reviewed before submission. |
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Description
In the collnet transport,
struct sharedResourcessizescollNetListenCommsandcollNetCommsbyMAXCHANNELS(64), but indexes them bynetDev, which ranges over[0, NCCL_MAX_NETDEVS)(128). The samenetDevindexes the adjacentcommRefCount[NCCL_MAX_NETDEVS]in the very same struct, the siblingncclCollNetSharedRes::proxyAppend[2*NCCL_MAX_NETDEVS], and the regular-netncclProxyProgressState::netComms[NCCL_MAX_NETDEVS]— all sized byNCCL_MAX_NETDEVS. So on a host where the selected collnetnetDevindex is>= 64,sharedListen/sharedConnect/sharedFreeread and writecollNetComms[netDev]/collNetListenComms[netDev]past the end of the 64-element arrays, into the neighbouringcommRefCountfield and beyond — a heap out-of-bounds access, not a clean assert.Related Issues
None.
Changes & Impact
src/transport/coll_net.cc(struct sharedResources): sizecollNetListenCommsandcollNetCommsbyNCCL_MAX_NETDEVS, matching theirnetDevindex domain (andcommRefCount/proxyAppend/netComms). ThencclCalloc-allocated struct grows by ~1 KB; indices 0–63 are byte-identical, only the previously-out-of-bounds 64–127 range is now valid storage. Non-breaking.Performance Impact
None.
Testing
make src.build;all_reduce_perfregression:Out of bounds values : 0 OK.static_assert(NCCL_MAX_NETDEVS <= MAXCHANNELS)would fail to compile) and from the three sibling arrays keyed on the samenetDev. The overrun only triggers with >64 collnet-capable NICs on one host, which cannot be reproduced on a single-GPU machine.