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Fix "flexible array member X not at end of struct" compiling with GCC-11 #348

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Fix "flexible array member X not at end of struct" compiling with GCC-11 #348

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19 changes: 9 additions & 10 deletions gloo/common/linux.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -192,29 +192,28 @@ const std::string& infinibandToBusID(const std::string& name) {
static int getInterfaceSpeedGLinkSettings(int sock, struct ifreq* ifr) {
#if LINUX_VERSION_CODE >= KERNEL_VERSION(4,6,0)
constexpr auto link_mode_data_nwords = 3 * 127;
struct {
struct ethtool_link_settings req;
__u32 link_mode_data[link_mode_data_nwords];
} ecmd;
constexpr auto bufsize = sizeof(struct ethtool_link_settings) + sizeof(__u32)*link_mode_data_nwords;
char buf[bufsize];
struct ethtool_link_settings& ecmd = *(struct ethtool_link_settings*)buf;
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IMO this is technically a strict aliasing violation and hence UB. At least buf needs to be aligned as struct ethtool_link_settings and possibly a placement new needs to be used.

Idea: Using an union to enforce the size might work:

  constexpr auto bufsize = sizeof(struct ethtool_link_settings) + sizeof(__u32)*link_mode_data_nwords;
  union {
    struct ethtool_link_settings req;
    char buf[bufsize];
  } ecmd;

int rv;

ifr->ifr_data = (__caddr_t)&ecmd;
memset(&ecmd, 0, sizeof(ecmd));
ecmd.req.cmd = ETHTOOL_GLINKSETTINGS;
memset(buf, 0, bufsize);
ecmd.cmd = ETHTOOL_GLINKSETTINGS;

rv = ioctl(sock, SIOCETHTOOL, ifr);
if (rv < 0 || ecmd.req.link_mode_masks_nwords >= 0) {
if (rv < 0 || ecmd.link_mode_masks_nwords >= 0) {
return SPEED_UNKNOWN;
}

ecmd.req.cmd = ETHTOOL_GLINKSETTINGS;
ecmd.req.link_mode_masks_nwords = -ecmd.req.link_mode_masks_nwords;
ecmd.cmd = ETHTOOL_GLINKSETTINGS;
ecmd.link_mode_masks_nwords = -ecmd.link_mode_masks_nwords;
rv = ioctl(sock, SIOCETHTOOL, ifr);
if (rv < 0) {
return SPEED_UNKNOWN;
}

return ecmd.req.speed;
return ecmd.speed;
#else
(void)sock;
(void)ifr;
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